# Defeasible Reasoning

*First published Fri Jan 21, 2005; substantive revision Tue Jan 25, 2005*

Reasoning is *defeasible* when the corresponding argument is
rationally compelling but not deductively valid. The truth of the
premises of a good defeasible argument provide support for the
conclusion, even though it is possible for the premises to be true and
the conclusion false. In other words, the relationship of support
between premises and conclusion is a tentative one, potentially
defeated by additional information. Philosophers have studied the
nature of defeasible reasoning since Aristotle's analysis of
*dialectical reasoning* in the *Topics* and the
*Posterior Analytic*, but the subject has been studied with
unique intensity over the last forty years, largely due to the
interest it attracted from the artificial intelligence movement in
computer science. There are have been two approaches to the study of
reasoning: treating it either as a branch of epistemology (the study
of knowledge) or as a branch of logic. In recent work, the term
*defeasible reasoning* has typically been limited to inferences
involving rough-and-ready, exception-permitting generalizations, that
is, inferring what has or will happen on the basis of what
*normally* happens. This narrower sense of *defeasible
reasoning*, which will be the subject of this article, excludes
from the topic the study of other forms of non-deductive reasoning,
including inference to the best explanation, abduction, analogical
reasoning, and scientific induction. This exclusion is to some extent
artificial, but it reflects the fact that the formal study of these
other forms of non-deductive reasoning remains quite rudimentary.

- 1. History
- 2. Applications and Motivation
- 3. Varieties of Approaches
- 4. Epistemological Approaches
- 5. Logical Approaches
- 5.1 Relations of Logical Consequence
- 5.2 Metalogical Desiderata
- 5.3 Default Logic
- 5.4 Nonmonotonic Logic I and Autoepistemic Logic
- 5.5 Circumscription
- 5.6 Preferential Logics
- 5.7 Logics of Extreme Probabilities
- 5.8 Fully Expressive Languages: Conditional Logics and Higher-Order Probabilities
- 5.9 Objections to Nonmonotonic Logic

- Causation and Defeasible Reasoning
- Bibliography
- Other Internet Resources
- Related Entries

## 1. History

Defeasible reasoning has been the subject of study by both philosophers and computer scientists (especially those involved in the field of artificial intelligence). The philosophical history of the subject goes back to Aristotle, while the field of artificial intelligence has greatly intensified interest in it over the last forty years.

### 1.1 Philosophy

According to Aristotle, deductive logic (especially in the form of
the syllogism) plays a central role in the articulation of scientific
understanding, deducing observable phenomena from definitions of
natures that hold universally and without exception. However, in the
practical matters of every day life, we rely upon generalizations that
hold only “for the most part”, under normal circumstances, and the
application of such common sense generalizations involves merely
*dialectical* reasoning, reasoning that is defeasible and falls
short of deductive validity. Aristotle lays out a large number and
great variety of examples of such reasoning in his work entitled the
*Topics*.

Investigations in logic after Aristotle (from later antiquity through the twentieth century) seem to have focused exclusively on deductive logic. This continued to be true as the predicate logic was developed by Peirce, Frege, Russell, Whitehead and others in the late nineteenth and early twentieth centuries. With the collapse of logical positivism in the mid twentieth century (and the abandonment of attempts to treat the physical world as a logical construction from facts about sense data), new attention was given to the relationship between sense perception and the external world. Roderick Chisholm (Chisholm 1957; Chisholm 1966) argued that sensory appearances give good, but defeasible, reasons for believing in corresponding facts about the physical world. If I am “appeared to redly” (have the sensory experience as of being in the presence of something red), then, Chisholm argued, I may presume that I really am in the presence of something red. This presumption can, of course, be defeated, if, for example, I learn that my environment is relevantly abnormal (for instance, all the ambient light is red).

John L. Pollock developed Chisholm's idea into a theory of *prima
facie reasons* and *defeaters* of those reasons (Pollock
1967; Pollock 1979; Pollock 1974). Pollock distinguished between two
kinds of defeaters of a defeasible inference: *rebutting
defeaters* (which give one a prima facie reason for believing the
denial of the original conclusion) and *undercutting defeaters*
(which give one a reason for doubting that the usual relationship
between the premises and the conclusion hold in the given case).
According to Pollock, a conclusion is warranted, given all of one's
evidence, if it is supported by an ultimately undefeated argument whose
premises are drawn from that evidence.

### 1.2 Artificial Intelligence

As the subdiscipline of artificial intelligence took shape in the
1960's, pioneers like John M. McCarthy and Patrick J. Hayes soon
discovered the need to represent and implement the sort of defeasible
reasoning that had been identified by Aristotle and Chisholm. McCarthy
and Hayes (McCarthy and Hayes 1969) developed a formal language they
called the “situation calculus”, for use by expert systems
attempting to model changes and interactions among a domain of objects
and actors. McCarthy and Hayes encountered what they called the
*frame problem*: the problem of deciding which conditions will
*not* change in the wake of an event. They required a
defeasible principle of inertia: the presumption that any given
condition will not change, unless required to do so by actual events
and dynamic laws. In addition, they encountered the *qualification
problem*: the need for a presumption that an action can be
successfully performed, once a short list of essential prerequisites
have been met. McCarthy (McCarthy 1977, 1038-1044) suggested that the
solution lay in a logical principle of *circumscription*: the
presumption that the actual situation is as unencumbered with
abnormalities and oddities (including unexplained changes and
unexpected interferences) as is consistent with our knowledge of
it. (McCarthy 1982; McCarthy 1986) In effect, McCarthy suggests that
it is warranted to believe whatever is true in all the
*minimal* (or otherwise *preferred*) models of one's
initial information set.

In the early 1980's, several systems of defeasible reasoning were
proposed by others in the field of artificial intelligence: Ray
Reiter's default logic (Reiter 1980; Etherington & Reiter 1983,
104-108), McDermott and Doyle's Non-Monotonic Logic I (McDermott and
Doyle, 1982), Robert C. Moore's Autoepistemic Logic (Moore 1985), and
Hector Levesque's formalization of the “all I know”
operator (Levesque 1990). These early proposals involved the search
for a kind of *fixed point* or cognitive equilibrium. Special
rules (called *default rules* by Reiter) permit drawing certain
conclusions so long as these conclusions are consistent with what one
knows, including all that one knows on the basis of these very default
rules. In some cases, no such fixed point exists, and, in others,
there are multiple, mutually inconsistent fixed points. In addition,
these systems were procedural or computational in nature, in contrast
to the semantic characterization of warranted conclusions (in terms of
preferred models) in McCarthy's circumscription system. Later work in
artificial intelligence has tended to follow McCarthy's lead in this
respect.

## 2. Applications and Motivation

Philosophers and theorists of artificial intelligence have found a
wide variety of applications for defeasible reasoning. In some cases,
the defeasibility seems to be grounded in some aspect of the subject
or the context of communication, and in other cases in facts about the
objective world. The first includes defeasible rules as communicative
or representational conventions and *autoepistemic* (reasoning
about one's own knowledge and lack of knowledge). The latter, the
objective sources of defeasibility, include defeasible obligations,
defeasible laws of nature, induction, abduction, and Ockham's razor
(the presumption that the world is as uncomplicated as possible).

### 2.1 Defeasibility as a Convention of Communication

Much of John McCarthy's early work in artificial intelligence
concerned the interpretation of stories and puzzles (McCarthy and Hayes
1969; McCarthy 1977). McCarthy found that we often make assumptions
based on what is not said. So, for example, in a puzzle about safely
crossing a river by canoe, we assume that there are no bridges or other
means of conveyance available. Similarly, when using a database to
store and convey information, the information that, for example, no
flight is scheduled at a certain time is represented simply by
*not* listing such a flight. Inferences based on these
conventions are defeasible, however, because the conventions can
themselves be explicitly abrogated or suspended.

### 2.2 Autoepistemic Reasoning

Robert C. Moore (Moore 1985) pointed out that we sometimes infer
things about the world based on our *not* knowing certain
things. So, for instance, I might infer that I do not have a sister,
since, if I did, I would certainly know it, and I do not in fact know
that I have a sister. Such an inference is, of course, defeasible,
since if I subsequently learn that I have a sister, after all, the
basis for the original inference is nullified.

### 2.3 Semantics for Generics and the Progressive

Generic terms (like *birds* in *Birds fly*) are
expressed in English by means of bare common noun phrases (without
determiner). Adverbs like *normally* and *typically* are
also indicators of generic predication. As Asher and Pelletier (Asher
and Pelletier 1997) have argued, the semantics for such sentences seems
to involve intentionality: a generic sentence can be true even if the
majority of the kind, or even all of the kind, fail to conform to the
generalization. It can be true that birds fly even if, as a result of a
freakish accident, all surviving birds are abnormally flightless. A
promising semantic theory for the generic is to represent generic
predication by means of a defeasible rule or conditional.

The progressive verb involves a similar kind of intentionality.
(Asher 1992) If Jones *is crossing the street*, then it would
normally be the case that Jones *will succeed* in crossing the
street. However, this inference is clearly defeasible: Jones might
be hit by a truck midway across and never complete the crossing.

### 2.4 Defeasible Obligations

Philosophers have, for quite some time, been interested in
defeasible obligations, which give rise to defeasible inferences about
what we are, all things considered, obliged to do. David Ross, in 1930,
discussed the phenomena of *prima facie* obligations (Ross
1930). The existence of a prima facie obligation gives one good, but
defeasible grounds, for believing that one ought to fulfill that
obligation. When formal *deontic logic* was developed by
Chisholm and others in the 1960s (Chisholm 1963), the use of classical
logic gave rise to certain paradoxes, such as Chisholm's paradox of
contrary-to-duty imperatives. These paradoxes can be resolved by
recognizing that the inference from imperative to actual duty is a
defeasible one (Asher and Bonevac 1996; Nute 1997).

### 2.5 Defeasible Laws of Nature

Philosophers David M. Armstrong and Nancy Cartwright have argued
that the actual laws of nature are *oaken* rather than
*iron* (to use Armstrong's terms). (Armstrong 1983; Armstrong
1997, 230-231; Cartwright 1983). Oaken laws admit of exceptions: they
have tacit *ceteris paribus* (other things being equal) or
*ceteris absentibus* (other things being absent) conditions. As
Cartwright points out, an inference based on such a law of nature is
always defeasible, since we may discover that additional
*phenomenological factors* must be added to the law in question
in special cases.

There are several reasons to think that deductive logic is not an adequate tool for dealing with this phenomenon. In order to apply deduction to the laws and the initial conditions, the laws must be represented in a form that admits of no exceptions. This would require explicitly stating each potentially relevant condition in the antecedent of each law-stating conditional. This is impractical, not only because it makes the statement of each and every law extremely cumbersome, but also because we know that there are many exceptional cases that we have not yet encountered and may not be able to imagine. Defeasible laws enable us to express what we really know to be the case, rather than forcing us to pretend that we can make an exhaustive list of all the possible exceptions.

### 2.6 Defeasible Principles in Metaphysics and Epistemology

Many classical philosophical arguments, especially those in the perennial philosophy that endured from Plato and Aristotle to the end of scholasticism, can be fruitfully reconstructed by means of defeasible logic. Metaphysical principles, like the laws of nature, may hold in normal cases, while admitting of occasional exceptions. The principle of causality, for example, that plays a central role in the cosmological argument for God's existence, can plausibly construed as a defeasible generalization (Koons 2001).

As discussed above (in section 1.1), prima facie reasons and defeaters of those reasons play a central role in contemporary epistemology, not only in relation to perceptual knowledge, but also in relation to every other source of knowledge: memory, imagination (as an indicator of possibility) and testimony, at the very least. In each cases, an impression or appearance provides good but defeasible evidence of a corresponding reality.

### 2.7 Occam's Razor and the Assumption of a “Closed World"

Prediction always involves an element of defeasibilty. If one predicts
what will, or what would, under some hypotheis, happen, one must
presume that there are no unknown factors that might interfere with
those factors and conditions that are known. Any prediction can be
upset by such unanticipated interventions. Prediction thus proceeds
from the assumption that the situation as modeled constitutes a
*closed world*: that nothing outside that situation could
intrude in time to upset one's predictions. In addition, we seem to
presume that any factor that is not known to be causally relevant is
in fact causally irrelevant, since we are constantly encountering new
factors and novel combinations of factors, and it is impossible to
verify their causal irrelevance in advance. This closed-world
assumption is one of the principal motivations for McCarthy's logic of
circumscription (McCarthy 1982; McCarthy 1986).

## 3. Varieties of Approaches

We can treat the study of defeasible reasoning either as a branch of
epistemology (the theory of knowledge), or as a branch of logic. In the
epistemological apporach, defeasible reasoning is studied as a form of
inference, that is, as a process by which we add to our stock of
knowledge. The epistemological approach is concerned with the
transmission of *warrant*, with the question of when an
inference, starting with justified or warranted beliefs, produces a new
belief that is also warranted. This approach focuses explicitly on the
norms of belief change.

In contrast, a logical approach to defeasible reasoning fastens on a
relationship between propositions or possible bodies of information.
Just as deductive logic consists of the study of a certain
*consequence relation* between propositions or sets of
propositions (the relation of valid implication), so defeasible (or
*nonmonotonic*) logic consists of the study of a different kind
of consequence relation. Deductive consequence is monotonic: if a set
of premises logically entails a conclusion, than any superset (any set
of premises that includes all of the first set) will also entail that
some conclusion. In contrast, defeasible consequence is nonmonotonic. A
conclusion follows defeasibly or nonmonotonically from a set of
premises just in case it is true in *nearly all* of the models
that verify the premises, or in the *most normal* models that
do.

The two approaches are related. In particular, a logical theory of
defeasible consequence will have epistemological consequences. It is
presumably true that an ideally rational thinker will have a set of
beliefs that are closed under defeasible, as well as deductive,
consequence. However, a logical theory of defeasible consequence would
have a wider scope of application than a merely epistemological theory
of inference. Defeasible logic would provide a mechanism for engaging
in *hypothetical* reasoning, not just reasoning from actual
beliefs.

Conversely, as David Makinson and Peter Gärdenfors have pointed
out (Makinson and Gärdenfors 1991, 185-205), an epistemological
theory of belief change can be used to define a set of nonmonotonic
consequence relations (one relation for each initial belief state). We
can define the consequence relation α
β, for a
given set of beliefs *T*, as holding just in case the result of adding
belief α to *T* would include belief in β. However, on this
approach, there would be many distinct nonmonotonic consequence
relations, instead of a single perspective-independent one.

## 4. Epistemological Approaches

There are have been three versions of the epistemological approach,
each of which attempts to define how an cognitively ideal agent
arrives at warranted conclusions, given an initial input. The first
two of these, John L. Pollock's theory of defeasible reasoning and the
theory of semantic inheritance networks, are explicitly computational
in nature. They take as input a complex, structured state,
representing the data available to the agent, and they define a
procedure by which new conclusions can be warranted. The third
approach, based on the theory of belief change (the AGM model)
developed by Alchourrón, Gärdenfors and Makinson
(Alchourrón, Gärdenfors and Makinson 1982), instead lays
down a set of conditions that an ideal process of belief change ought
to satisfy. The AGM model can be used to define a nonmonotonic
consequence relation that is temporary and local. This can represent
reasoning that is hypothetically or counterfactually defeasible, in
the sense that what “follows” from a conjunctive
proposition (*p* & *q*) need not be a superset of
what “follows” from *p* alone.

### 4.1 Formal Epistemology

John Pollock's approach to defeasible reasoning consists of
enumerating a set of rules that are constructive and effectively
computable, and that aim at describing how an ideal cognitive agent
builds up a rich set of beliefs, beginning with a relatively sparse
data set (consisting of beliefs about immediate sensory appearances,
apparent memories, and such things). The inferences involved are not,
for the most part, deductive. Instead, Pollock defines, first, what it
is for one belief to be a *prima facie reason* for believing
another proposition. In addition, Pollock defines what it is for one
belief, say in *p*, to be a *defeater* for *q* as
a prima facie reason for *r*. In fact. Pollock distinguishes
two kinds of defeaters: *rebutting defeaters*, which are
themselves prima facie reasons for believing the negation of the
conclusion, and *undercutting defeaters*, which provide a
reason for doubting that q provides any support, in the actual
circumstances, for *r*. (Pollock 1987, 484) A belief is
*ultimately warranted* in relation to a data set (or
*epistemic basis*) just in case it is supported by some
ultimately undefeated argument proceeding from that epistemic
basis.

In his most recent work (Pollock 1995), Pollock uses a directed
graph to represent the structure of an ideal cognitive state. Each
directed link in the network represent the first node's being a prima
face reason for the second. The new theory includes an account of
*hypothetical*, as well as categorical reasoning, since each
node of the graph includes a (possibly empty) set of hypotheses.
Somewhat surprisingly, Pollock assumes a principle of monotonicity with
respect to hypotheses: a belief that is warranted relative to a set of
hypotheses is also warranted with respect to any superset of
hypotheses. Pollock also permits conditionalization and reasoning by
cases.

An argument is *self-defeating* if it supports a defeater for
one of its own defeasible steps. Here is an interesting example: (1)
Robert says that the elephant beside him looks pink. (2) Robert's color
vision becomes unreliable in the presence of pink elephants.
Ordinarily, belief 1 would support the conclusion that the elephant is
pink, but this conclusion undercuts the argument, thanks to belief 2.
Thus, the argument that the elephant is pink is self-defeating.
Pollock argues that all self-defeating arguments should be rejected,
and that they should not be allowed to defeat other arguments. In
addition, a set of nodes can experience mutual destruction or
*collective defeat* if each member of the set is defeated by
some other member, and no member of the set is defeated by an
undefeated node that is outside the set.

In formalizing the undercutting rebuttal, Pollock introduces a new
connective,
,
where
*p*
*q*
means that it is not the case that *p* wouldn't be true unless
*q* were true. Pollock uses rules, rather than conditional
propositions, to express the prima facie relation. If he had, instead,
introduced a special connective ⇒, with *p* ⇒
*q* meaning that *p* would be a prima facie reason for
*q*, then undercutting defeaters could be represented by means
of negating this conditional. To express the fact that *r* is
an undercutting defeater of *p* as a prima facie reason for
*q*, we could state both that (*p* ⇒ *q*)
and ¬((*p* & *r*) ⇒ *q*).

In the case of conflicting prima facie reasons, Pollock rejects the
principle of *specificity*, a widely accepted principle
according to which the defeasible rule with the more specific
antecedent takes priority over conflicting rules with less specific
antecedents. Pollock does, however, accept a special case of
specificity in the area of statistical syllogisms with projectible
properties. (Pollock 1995, 64-66) So, if I know that most *A*'s
are *B*'s, and the most *AC*'s are not *B*'s,
then I should, upon learning that individual *b* is both
*A* and *C*, give priority to the *AC*
generalization over the *A* generalization (concluding that
*b* is not a *B*).

Pollock's theory of warrant is intended to provide normative rules for belief, of the form: if you have warranted beliefs that are prima facie reasons for some further belief, and you have no ultimately undefeated defeaters for those reasons, then that further belief is warranted and should be believed. For more details of Pollock's theory, see the following supplementary document:

John Pollock's System

Wolfgang Spohn (Spohn 2002) has argued that Pollock's system is
*normatively defective* because, in the end, Pollock has no
normative standard to appeal to, other than ad hoc intuitions about
how a reasonable person would respond to this or that cognitive
situation. Spohn suggests that, with respect to the state of
development of the study of defeasible reasoning, Pollock's theory
corresponds to C. I. Lewis's early investigations into modal
logic. Lewis suggested a number of possible axiom systems, but lacked
an adequate semantic theory that could provide an independent check on
the correctness or completeness of any given list (of the kind that
was later provided by Kripke and Kanger). Analogously, Spohn argues
that Pollock's system is in need of a unifying normative
standard. This very same criticism can be lodged, with equal justice,
against a number of other theories of defeasible reasoning, including
semantic inheritance networks and default logic.

### 4.2 Semantic Inheritance Networks

The system of semantic inheritance networks, developed by Horty, Thomason and Touretzky (Horty, Thomason and Touretzky 1990), is similar to Pollock's system. Both represent cognitive states by means of directed graphs, with links representing defeasible inferences. The semantic inheritance network theory has a intentionally narrower scope: the initial nodes of the network represent particular individuals, and all non-initial nodes represent kinds, categories or properties. A link from an initial (individual) node to a category node represents simply predication: that Felix (initial node) is a cat (category node), for example. Links between category nodes represent defeasible or generic inclusion: that birds (normally or usually) are flying things. To be more precise, there are both positive (“is a”) and negative (“is not a”) links. The negative links are usually reprented by means of a slash through the body of the arrow.

Semantic inheritance networks differ from Pollock's system in two
important ways. First, they cannot represent one fact's constituting
an *undercutting* defeater of an inference, although they can
represent *rebutting* defeaters. For example, they do not allow
an inference from the apparent color of an elephant to its actual
color to be undercut by the information that my color vision is
unreliable, unless I have information about the actual color of the
elephant that contradicts its apparent color. Secondly, they do
incorporate the principle of specificity (the principle that rules
with more specific antecedents take priority in case of conflict) into
the very definition of a warranted conclusion. In fact, in contrast
to Pollock, the semantic inheritance approach gives priority to rules
whose antecedents are weakly or defeasibly more specific. That is, if
the antecedent of one rule is defeasibly linked to the antecedent of a
second rule, the first rule gains priority. For example, if Quakers
are typically pacifists, then, when reasoning about a Quaker pacifist,
rules pertaining to Quakers would override rules pertaining to
pacifists. For the details of semantic inheritance theory, see the
following supplementary document:

Semantic Inheritance Networks.

David Makinson (Makinson 1994) has pointed out that semantic network theory is very sensitive to the form in which defeasible information is represented. There is a great difference between having a direct link between two nodes and having a path between the two nodes being supported by the graph as a whole. The notion of preemption gives special powers to explicitly given premises over conclusions. Direct links always take priority over longer paths. Consequently, inheritance networks lack two desirable metalogical properties: cut and cautious monotony (which will be covered in more detail in the section on Logical Approaches).

- Cut: If
*G*is a subgraph of*G*′, and every link in*G*′ corresponds to a path supported by*G*, then every path supported by*G*is also supported by*G*′. - Cautious Monotony: If
*G*is a subgraph of*G*′, and every link in*G*′ corresponds to a path supported by*G*, then every path supported by*G*′ is also supported by*G*.

Cumulativity (Cut plus Cautious Monotony) corresponds to reasoning by
lemmas or subconclusions. The Horty-Thomason-Touretzky system does
satisfy special cases of Cut and Cautious Monotony: if *A* is
an atomic statement (a link from an individual to a category), then if
graph *G* supports *A*, then for any statement
*B*, *G* ∪ {*A*} supports *B* if and
only if *G* supports *B*.

Another form of inference that is not supported by semantic inheritance networks is that of reasoning by cases or by dilemma. In addition, semantic networks do not license modus-tollens-like inferences: from the fact that birds normally fly and Tweety does not fly, we are not licensed to infer that Tweety is not a bird. (This feature is also lacking in Pollock's system.)

### 4.3 Belief Revision Theory

Alchourrón, Gärdenfors and Makinson (Alchourrón,
Gärdenfors and Makinson 1982) developed a formal theory of belief
revision and contraction, drawing largely on Willard van Orman Quine's
model of the *web of belief* (Quine and Ullian 1970). The
cognitive agent is modelled as believing a set of propositions that
are ordered by their degree of entrenchment. This model provides the
basis for a set of normative constraints on belief contraction
(subtracting a belief) and belief revision (adding a new belief that
is inconsistent with the original set). When a belief is added that is
logically consistent with the original belief set, the agent is
supposed to believe the logical closure of the original set plus the
new belief. When a belief is added that is inconsistent with the
original set, the agent retreats to the most entrenched of the maximal
subsets of the set that are consistent with the new belief, adding the
new proposition to that set and closing under logical consequence. For
the axioms of the AGM model, see the following supplementary
document:

AGM Postulates

AGM belief revision theory can be used as the basis for a system of
defeasible reasoning or nonmonotonic logic, as Gärdenfors and
Makinson have recognized (Makinson and Gärdenfors 1991). If *K* is
an epistemic state, then a nonmonotonic consequence relation
can be defined
as follows: *A*
*B* iff *B* ∈ *K***A*. Unlike
Pollock's system or semantic inheritance networks, this defeasible
consequence relation depends upon a background epistemic state. Thus,
the belief revision approach gives rise, not to a single nonmonotonic
consequence relation, but to family of relations. Each background
state *K* gives rise to its own characteristic consequence
relation.

One significant limitation of the belief-revision approach is that
there is no representation in the object-language of a defeasible or
default rule or conditional (that is, of a conditional of the form
*If p, then normally q* or *That p would be a prima facie
reason for accepting that q*). In fact, Gärdenfors
(Gärdenfors 1978; Gärdernfors 1986) proved that no
conditional satisfying the Ramsey test can be added to the AGM system
without trivializing the revision
relation.^{[1]}
(A conditional ⇒ satisfies the Ramsey test just in case, for
every epistemic state *K*, *K* includes (*A*
⇒ *B*) iff *K***A* includes *B*.)

Since the AGM system cannot include conditional beliefs, it cannot elucidate the question of what logical relationships hold between conditional defaults.

The lack of a representation of conditional beliefs is closely
connected to another limitation of the AGM system: its inability to
model repeated or *iterated* belief revision. The input to a
belief change is an epistemic state, consisting both of a set of
propositions believed and an entrenchment relation on that set. The
output of an AGM revision, in contrast, consists simply of a set of
beliefs. The system provides no guidance on the question of what would
be the result of revising an epistemic state in two or more steps. If
the entrenchment relation could be explicitly represented by means of
conditional propositions, then it would be possible to define the new
entrenchment relation that would result from a single belief revision,
making iterated belief revision representable. A number of proposals
along these lines have been made. The difficulty lies in defining
exactly what would constitute a *minimal* change in the
relative entrenchment or epistemic ranking of a set of beliefs. To
this point, no clear consensus has emerged on this question. (See
Spohn 1988; Nayak 1994; Wobcke 1995; Bochman, 2001.)

On the larger question of the relation between belief revision and
defeasible reasoning, there are two possibilities: that a theory of
defeasible reasoning should be grounded in a theory of belief revision,
and that a theory of belief revision should be grounded in a theory of
defeasible reasoning. The second view has been defended by John Pollock
(Pollock 1987; Pollock 1995) and by Hans Rott (Rott 1989). On this
second view, we must make a sharp distinction between basic or
foundational beliefs on the one hand and inferred or derived beliefs on
the other. We can then model belief change on the assumption that new
beliefs are added to the foundation (and are logically consistent with
the existing set of those beliefs). Beliefs can be added which are
inconsistent with previous inferred beliefs, and the new belief state
consists simply in the closure of the new foundational set under the
relation of defeasible consequence. On such an approach, default
conditionals can be explicitly represented among the agent's beliefs.
Gärdenfors's triviality result is then avoided by rejecting one of
the assumptions of the theorem, *preservation*:

Preservation: If ¬AK, thenK⊆K*A.

From the perspective that uses defeasible reasoning to define belief
revision, there is no good reason to accept Preservation. One can add
a belief that is consistent with what one already believes and thereby
*lose* beliefs, since the new information might be an
undercutting defeater to some defeasible inference that had been
successful.

## 5. Logical Approaches

Logical approaches to defeasible reasoning treat the subject as a part
of logic: the study of *nonmonotonic* consequence relations (in
contrast to the monotonicity of classical logic). These relations are
defined on propositions, not on the beliefs of an agent, so the focus
is not on epistemology per se, although a theory of nonmonotonic logic
will certainly have implications for epistemology.

### 5.1 Relations of Logical Consequence

A consequence relation is a mathematical relation that models what
follows logically from what. Consequence relations can be defined in a
variety of ways: Hilbert, Tarski and Scott relations. A Hilbert
consequence relation is a relation between pairs of formulas, a Tarski
relation is a relation between sets of formulas (possibly infinite) and
individual formulas, and a Scott relation is a relation between two
sets of formulas. In the case of Hilbert and Tarski relations, A
*B* or Γ
*B* mean that the formula *B* follows from formula
*A* or from set of formulas Γ. In the case of Scott
consequence relations, Γ
Δ means that the joint truth of all the members of Γ
implies (in some sense) the truth of at least one member of
Δ. To this point, studies of nonmonotonic logic have defined
nonmonotonic consequence relations in the style of Hilbert or Tarski,
rather than Scott.

A (Tarski) consequence relation is *monotonic* just in case it
satisfies the following condition, for all formulas *p* and all
sets Γ and Δ:

Monotonicity: If Γp, then Γ ∪ Δp.

Any consequence relation that fails this condition is
*nonmonotonic*. A relation of defeasible consequence clearly
must be nonmonotonic, since a defeasible inference can be defeated by
adding additional information that constitutes a rebutting or
undercutting defeater.

### 5.2 Metalogical Desiderata

Once monotonicity is given up, the question arises: why call the
relation of defeasible consequence a *logical consequence*
relation at all? What properties do defeasible consequence and
classical logical consequence have in common, that would justify
treating them as sub-classes of the same category? What justifies
calling nonmonotonic consequence *logical*?

To count as *logical*, there are certain minimal properties
that a relation must satisfy. First, the relation ought to permit
reasoning by lemmas or subconclusions. That is, if a proposition
*p* already follows from a set Γ, then it should make no
difference to add *p* to Γ as an additional
premise. Relations that satisfy this condition are called
*cumulative*. Cumulative relations satisfy the following two
conditions (where “*C*(Γ)” represents the set of
defeasible consequences of Γ):

Cut: If Γ ⊆ Δ ⊆C(Γ), thenC(Δ) ⊆C(Γ).

Cautious Monotony: If Γ ⊆ Δ ⊆C(Γ), thenC(Γ) ⊆C(Δ).

In addition, a defeasible consequence relation ought to be
*supraclassical*: if *p* follows from *q* in
classical logic, then it ought to be included in the defeasible
consequences of *q* as well. A formula *q* ought to
count as an (at least) defeasible consequence of itself, and anything
included in the content of *q* (any formula *p* that
follows from *q* in classical logic) ought to count as a
defeasible consequence of *q* as well. Moreover, the defeasible
consequences of a set Γ ought to depend only on the content of
the formulas in Γ, not in how that content is
represented. Consequently, the defeasible consequence relation ought
to treat Γ and the classical logical closure of Γ (which
I'll represent as “*Cn*(Γ)”) in exactly the same
way. A consequence relation that satisfies these two conditions is
said to satisfy *full absorption* (see Makinson 1994, 47).

Full Absorption:Cn(C(Γ)) =C(Γ) =C(Cn(Γ))

Finally, a genuinely logical consequence relation ought to enable us
to reason by cases. So, it should satisfy a principle called
distribution: if a formula *p* follows defeasibly from both
*q* and *r*, then it ought to follow from their disjunction. (To
require the converse principle would be to reinstate monotonicity.)
The relevant principle is this:

Distribution:C(Γ) ∩C(Δ) ⊆C(Cn(Γ) ∩Cn(Δ)).

Consequence relations that are cumulative, strongly absorptive and
distributive satisfy a number of other desirable properties, including
*conditionalization*: If a formula *p* is a defeasible
consequence of Γ ∪ {*q*}, then the material
conditional (*q* → *p*) is a defeasible consequence
of Γ alone. In addition, such logics satisfy the property of
*loop*: if *p*_{1}
*p*_{2}
… *p*_{n-1}
*p*_{n} (where
“" represents the
defeasible consequence relation), then the defeasible consequences of
*p*_{i} and *p*_{j}
are exactly the same, for any *i* or
*j*.^{[2]}

There are three further conditions that have been much discussed in
the literature, but whose status remains controversial:
*disjunctive rationality*, *rational monotony* and
*consistency preservation*.

Disjunctive Rationality: If Γ ∪ {p}r, and Γ ∪ {q}r, then Γ ∪ {(p q)}r.

Rational Monotony: If ΓA, then either Γ ∪ {B}Aor Γ ¬B.

Consistency Preservation: If Γ is classically consistent, then so isC(Γ) (the set of defeasible consequences of Γ).

All three properties seem desirable, but they set a very hight standard for the defeasible reasoner.

### 5.3 Default Logic

Ray Reiter's default logic (Reiter 1980; Etherington and Reiter 1983) was part of the first generation of defeasible systems developed in the field of artificial intelligence. The relative ease of computing default extensions have made it one of the more popular systems.

Reiter's system is based on the use of *default rules*. A
default rule consists of three formulas: the *prerequisite*, the
*justification*, and the *consequent*. If one accepts the
prerequisite of a default rule, and the justification is consistent
with all one knows (including what one knows on the basis of the
default rules themselves), then one is entitled to accept the
consequent. The most popular use of default logic relies solely on
*normal defaults*, in which the justification and the consequent
are identical. Thus, a normal default of the form (*p*; *q*
*q*) allows one to infer *q* from *p*, so long
as *q* is consistent with one's endpoint (the
*extension* of the default theory).

A default theory consists of a set of formulas (the facts), together
with a set of default rules. An *extension* of a default theory
is a fixed point of a particular inferential process: an extension
*E* must be a consistent theory (a consistent set closed under
classical consequence) that contains all of the facts of the default
theory *T*, and, in addition, for each normal default
(*p* ⇒ *q*), if *p* belongs to *E*,
and *q* is consistent with *E*, then *q* must
belong to *E* also.

Since the consequence relation is defined by a fixed-point condition,
there are default theories that have no extension at all, and other
theories that have multiple, mutually inconsistent extensions. For
example, the theory consisting of the fact *p* and the pair of defaults
(*p* ; (*q* & *r*)
*q*) and (*q* ; ¬*r*
¬*r*) has no extension. If the first default is applied, then the
second must be, and if the second default is not applied, the first
must be. However, the conclusion of the second default contradicts
the prerequisite of the first, so the first cannot be applied if the
second is. There are many default theories that have multiple
extensions. Consider the theory consisting of the facts *q* and r and
the pair of defaults (*q* ; *p*
*p*) and (*r* ; ¬*p*
¬*p*). One or the other, but not both, defaults must be
applied.

Furthermore, there is no guarantee that if *E* and
*E*′ are both extensions of theory *T*, then the
intersection of *E* and *E*′ is also an extension
(the intersection of two fixed points need not be itself a fixed
point). Default logic is usually interpreted as a *credulous*
system: as a system of logic that allows the reasoner to select
*any* extension of the theory and believe all of the members of
that theory, even though many of the resulting beliefs will involve
propositions that are missing from other extensions (and may even be
contradicted in some of those extensions).

Default logic fails many of the tests for a logical relation that
were introduced in the previous section. It satisfied Cut and Full
Absorption, but it fails Cautious Monotony (and thus fails to be
cumulative). In addition, it fails Distribution, a serious limitation
that rules out reasoning by cases. For example, if one knows that Smith
is either Amish or Quaker, and both Quakers and Amish are normally
pacificists, one cannot infer that Smith is a pacificist. Default logic
also fails to represent Pollock's *undercutting defeaters*.
Finally, default logic does not incorporate any form of the principle
of *Specificity*, the principle that defaults with more specific
prerequisites ought, in cases of conflict, to take priority over
defaults with less specific prerequisites.

### 5.4 Nonmonotonic Logic I and Autoepistemic Logic

In both McDermott-Doyle's Nonmonotonic Logic I and Moore's
Autoepistemic logic (McDermott and Doyle, 1982; Moore, 1985; Konolige
1994), a modal operator *M* (representing a kind of epistemic
possibility) is used. Default rules take the following form:
((*p* & *Mq*) → *q*), that is, if
*p* is true and *q* is “possible” (in the
relevant sense), then *q* is also true. In both cases, the
extension of a theory is defined, as in Reiter's default logic, by
means of a fixed-point operation. *Mp* represents the fact that
¬*p* does not belong to the extension. For example, in
Moore's case, a set Δ is a *stable expansion* of a theory
Γ just in case Δ is the set of classical consequences of
the set Γ ∪ {¬*Mp*: *p* ∈ Δ}
∪ {*Mp*: *p*
Δ}. As in the case of Reiter's default logic,
some theories will lack a stable expansion, or have more than one. In
addition, these systems fail to incorporate *Specificity*.

### 5.5 Circumscription

In circumscription (McCarthy 1982; McCarthy 1986; Lifschitz 1988), one
or more predicates of the language are selected for minimization
(there is, in addition, a further technical question of which
predicates to treat as fixed and which to treat as variable). The
nonmonotonic consequences of a theory *T* then consist of all
the formulas that are true in every model of *T* that minimizes
the extensions of the selected predicates. One model *M* of
*T* is preferred to another, *M*', if and only if, for
each designated predicate *F*, the extension of *F* in
*M* is a subset of the extension of *F* in *M*',
and, for some such predicate, the extension in *M* is a
*proper subset* of the extension in *M*'.

The relation of circumscriptive consequence has all the desirable meta-logical properties. It is cumulative (satisfies Cut and Cautious Monotony), strongly absorptive and distributive. In addition, it satisfies Consistency Preservation, although not Rational Monotony.

The most critical problem in applying circumscription is that of
deciding on what predicates to minimize (there is, in addition, a
further technical question about which predicates to treat as fixed
and which as variable in extension). Most often what is done is to
introduce a family of *abnormality* predicates
*ab*_{1}, *ab*_{2}, etc. A default rule
then can be written in the form:
∀*x*((*F*(*x*) & ¬
*ab*_{i}(*x*) ) →
*G*(*x*)), where “→” is the ordinary
material conditional of classical logic. To derive the consequences of
a theory, all of the abnormality predicates are simultaneously
minimized. This simple approach fails to satisfy the principle of
Specificity, since each default is given its own, independent
abnormality predicate, and each are therefore treated with the same
priority. It is possible to add special rules for the prioritizing of
circumscription, but these are, of necessity, ad hoc and exogenous,
rather than a natural result of the definition of the consequence
relation.

Circumscription does have the capacity of representing the existence
of *undercutting defeaters*. Suppose that satisfying predicate
*F* provides a prima facie reason for supposing something to be
a *G*, and suppose that we use the abnormality predicate
*ab*_{1} in representing this default rule. We can
state that the predicate *H* provides an undercutting defeater
to this inference by simply adding the rule: ∀ *x*
(*H*(*x*) → *ab*_{1}(*x*)),
stating that all *H*'s are abnormal in respect number 1.

### 5.6 Preferential Logics

Circumscription is a special case of a wider class of defeasible
logics, the *preferential* logics (Shoham 1987). In preferential
logics, Γ
*p* iff *p* is true in all of the *most
preferred* models of Γ. In the case of circumscription, the
most preferred models are those that minimize the extension of certain
predicates, but many other kinds of preference relations can be used
instead, so long as the preference relations are transitive and
irreflexive (a strict partial order). A structure consisting of a set
of models of a propositional or first-order language, together with a
preference order on those models, is called a *preferential
structure*. The symbol
shall represent the preference relation.
*M*
*M*′ means that *M* is strictly preferred to
*M*′. A most preferred model is one that is
*minimal* in the ordering.

In order to give rise to a cumulative logic (one that satisfies Cut
and Cautious Monotony), we must add an additional condition to the
preferential structures, a Limit Assumption (also known as the
condition of *stopperedness* or *smoothness*:

Limit Assumption: Given a theoryT, andM, a non-minimal model ofT, there exists a modelM′ which is preferred toMand which is a minimal model ofT.

The Limit Assumption is satisfied if the preferential structure does not contain any infinite descending chains of more and more preferred models, with no minimal member. This is a difficult condition to motivate as natural, but without it, we can find preferential structures that give rise to nonmonotonic consequence relations that fail to be cumulative.

Once we have added the Limit Assumption, it is easy to show that any
consequence relation based upon a preferential model is not only
cumulative but also supraclassical, strongly absorptive and
distributive. Let's call such logics *preferential*. In fact,
Kraus, Lehmann and Magidor (Kraus, Lehmann and Magidor 1990; Makinson
1994, 77) proved the following representation theorem for preferential
logics:

Representation Theorem for Preferential Logics: if is a cumulative, supraclassical, strongly absorptive, and distributive consequence relation (i.e., a preferential relation) then there is a preferential structure satisfying the Limit Assumption such that for allfinitetheoriesT, the set of -consequences ofTis exactly the set of formulas true in every preferred model ofTin .^{[3]}

There are preferential logics that fail to satisfy consistency preservation, as well as disjunctive rationality and rational monotony:

Disjunctive Rationality: If Γ ∪ {p}r, and Γ ∪ {q}r, then Γ ∪ {(pq)}r.

Rational Monotony: If Γp, then either Γ ∪ {q}por Γ ¬q.

A very natural condition has been found by Kraus, Lehmann and
Magidor that corresponds to Rational Monotony: that of *ranked
models*. (No condition on preference structures has been found that
ensures disjunctive rationality without also ensuring rational
monotony.) A preferential structure
satisfies the Ranked Models condition just in case there is a
function *r* that assigns an ordinal number to each model in
such a way that *M*
*M*′ iff *r*(*M*) <
*r*(*M*'). Let's say that a preferential consequence
relation is a *rational* relation just in case it satisfies
Rational Monotony, and that a preferential structure is a
*rational* structure just in case it satisfies the ranked
models condition. Kraus, Lehmann and Magidor (Kraus, Lehmann and
Magidor 1990; Makinson 71-81) also proved the following representation
theorem:

Representation Theorem for Rational Logics: if is a rational consequence relation (i.e., a preferential relation that satisfies Rational Monotony) then there is a preferential structure satisfying the Limit Assumption and the Ranked Models Assumption such that for all finite theoriesT, the set of -consequences ofTis exactly the set of formulas true in every preferred model ofTin .

### 5.7 Logics of Extreme Probabilities

Lehmann and Magidor (Lehmann and Magidor 1992) noticed an
interesting coincidence: the metalogical conditions for preferential
consequence relations correspond exactly to the axioms for a logic of
conditionals developed by Ernest W. Adams (Adams
1975).^{[4]}
Adams's logic was based on a conditional, ⇒, intended to
represent a relation of very high conditional probability: (*p*
⇒ *q*) means that the conditional probability
*Pr*(*q*/*p*) is extremely close to 1. Adams used
the standard delta-epsilon definition of the calculus to make this
idea precise. Let us suppose that a theory *T* consists of a
set of conditional-free formulas (the facts) and a set of
probabilistic conditionals. A conclusion *p* follows defeasibly
from *T* if and only if every probability function satisfies
the following condition:

For every δ, there is an ε such that, if the probability of every fact in

Tis assigned a probability at least as high as 1 - ε, and every conditional inTis assigned a conditional probability at least as high as 1 - ε, then the probability of the conclusionpis at least 1 - δ.

The resulting defeasible consequence relation is a preferential relation. (It need not, however, be consistency-preserving.) This consequence relation also corresponds to a relation, 0-entailment, defined by Judea Pearl (Pearl 1990), as the common core to all defeasible consequence relations.

Lehmann and Magidor (1992) proposed a variation on Adams's idea.
Instead of using the delta-epsilon construction, they made use of
nonstandard measure theory, that is, a theory of probability functions
that can take values that are *infinitesimals* (infinitely
small numbers). In addition, instead of defining the consequence
relation by quantifying over *all* probability functions,
Lehmann and Magidor assume that we can select a single probability
function (representing something like the ideally rational or
objective probability). On their construction, a conclusion *p*
follows from *T* just in case the probability of *p* is
infinitely close to 1, on the assumption that the probabilities
assigned to members of *T* are infinitely close to 1. Lehmann and
Magidor proved that the resulting consequence relation is always not
only preferential: it is also *rational*. The logic defined by
Lehmann and Magidor also corresponds exactly to the theory of Popper
functions, another extension of probability theory designed to handle
cases of conditioning on propositions with infinitesimal probability
(see Harper 1976; Hawthorne 1998). For a brief discussion of Popper
functions, see the following supplementary document:

Popper Functions

In addition, Kraus, Lehmann and Magidor proved that, for every
preferential consequence relation
that is
probabilistically
admissible,^{[5]}
there is a unique rational consequence
relation
*
that
minimally extends it (that is, that the intersection of all the
rational consequence relations extending
is also a
rational consequence relation). This relation,
*,
is called the
*rational closure* of
.
To find the rational closure of a preferential
relation, one can perform the following operation on a preferential
structure that supports that relation: assign to each model in the
structure the smallest number possible, respecting the preference
relation. Judea Pearl also proposed the very same idea under the name
*1-entailment* or *System Z* (Pearl 1990).

A critical advantage to the Lehmann-Magidor-Pearl 1-entailment system
over Adams's epsilon-entailment lay in the way in which 1-entailment
handles irrelevant information. Suppose, for example, that we know
that birds fly (*B* ⇒ *F*), Tweety is a bird
(*B*) and Nemo is a whale (*W*). These premises do not
epsilon-entail *F* (that Tweety flies), since there is no
guarantee that a probability function assign a high probability to
*F*, given the *conjunction* of *B* and
*W*. In contrast, 1-entailment does give us the conclusion
*F*.

Moreover, 1-entailment satisfies a condition of *weak independence
of defaults*: conditionals with logically unrelated antecedents
can “fire” independently of each other: one can warrant a
conclusion even though we are given an explicit exception to the
other. Consider, for example, the following case: birds fly
(*B* ⇒ *F*), Tweety is a bird that doesn't fly
(*B* & ¬*F*), whales are large (*W*
⇒ *L*), and Nemo is a whale (*W*). These premises
1-entail that Nemo is large (*L*). In addition, 1-entailment
automatically satisfies the principle of Specificity: conditionals
with more specific antecedents are always given priority over those
with less specific antecedents.

There is another form of independence, *strong independence*,
that even 1-entailment fails to satisfy. If we are given one exception
to a rule involving a given antecedent, then we are unable to use any
conditional with the same antecedent to derive any conclusion
whatsoever. Suppose, for example, that we know that birds fly
(*B* ⇒ *F*), Tweety is a bird that doesn't fly
(*B* & ¬*F*), and birds lay eggs (*B*
⇒ *E*). Even under 1-entailment, the conclusion that
Tweety lays eggs (*E*) fails to follow. This failure to satisfy
Strong Independence is also known as *the Drowning Problem*
(since all conditionals with the same antecedent are
“drowned” by a single exception).

A consensus is growing that the Drowning Problem should not be
“solved” (see Pelletier and Elio 1994; Wobcke 1995, 85;
Bonevac, 2003, 461-462). Consider the following variant on the
problem: birds fly, Tweety is a bird that doesn't fly, and birds have
strong forelimb muscles. Here it seems we should refrain from
concluding that Tweety has strong forelimb muscles, since there is
reason to doubt that the strength of wing muscles is causally (and
hence, probabilistically) independent of capacity for flight. Once we
know that Tweety is an exceptional bird, we should refrain from
applying other conditionals with *Tweety is a bird* as their
antecedents, unless we know that these conditionals are independent of
flight, that is, unless we know that the conditional with the stronger
antecedent, *Tweety is a non-flying bird*, is also true.

Nonetheless, several proposals have been made for securing strong
independence and solving the Drowning Problem. Geffner and Pearl
(Geffner and Pearl 1992) proposed a system of *conditional
entailment*, a variant of circumscription, in which the preference
relation on models is defined in terms of the sets of defaults that
are satisfied. This enables Geffner and Pearl to satisfy both the
Specificity principle and Strong Independence. Another proposal is the
maximum entropy approach (Pearl 1988, 490-496; Goldszmidt, Morris and
Pearl, 1993; Pearl 1990). A theory *T*, consisting of defaults
Δ and facts *F*, entails *p* just in case the
probability of *p*, conditional on *F*, approaches 1 as
the probabilities associated with Δ approach 1, using the
entropy-maximizing^{[6]}
probability function that respects the defaults in Δ. The
maximum-entropy approaches satisfies both Specificity and Strong
Independence.

Every attempt to solve the drowning problem (including conditional
entailment and the maximum-entropy approach) come at the cost of
sacrificing cumulativity. Securing strong independence makes the
systems very sensitive to the exact *form* in which the default
information is stored. Consider, for example the following case: Swedes
are (normally) fair, Swedes are (normally) tall, Jon is a short Swede.
Conditional entailment and maximum-entropy entailment would permit the
conclusion that Jon is fair in this case. However, if we replace the
first two default conditionals by the single default, *Swedes are
normally both tall and fair*, then the conclusion no longer
follows, despite the fact that the new conditional is logically
equivalent to the conjunction of the two original conditionals.

Applying the logic of extreme probabilities to real-world defeasible reasoning generates an obvious problem, however. We know perfectly well that, in the case of the default rules we actually use, the conditional probability of the conclusion on the premises is nowhere near 1. For example, the probability that an arbitrary bird can fly is certainly not infinitely close to 1. This problem resembles that of using idealizations in science, such as frictionless planes and ideal gases. It seems reasonable to think that, in deploying the machinery of defeasible logic, we indulge in the degree of make-believe necessary to make the formal models applicable. Nonetheless, this is clearly a problem warranting further attention.

### 5.8 Fully Expressive Languages: Conditional Logics and Higher-Order Probabilities

With relatively few exceptions, the logical approaches to defeasible
reasoning developed so far put severe restrictions on the logical form
of propositions included in a set of premises. In particular, they
require the default conditional operator, ⇒, to have wide scope
in every formula in which it appears. Default conditionals are not
allowed to be nested within other default conditionals, or within the
scope of the usual Boolean operators of propositional logic (negation,
conjunction, disjunction, material conditional). This is a very severe
restriction and one that is quite difficult to defend. For example, in
representing *undercutting defeaters*, it would be very natural
to use a negated default conditional of the form ¬((*p*
& *q*) ⇒ *r*) to signify that *q*
defeats *p* as a prima facie reason for *r*. In
addition, it seems plausible that one might come gain
*disjunctive* default information: for example, that either
customers are gullible or salesman are wily.

Asher and Pelletier (Asher and Pelletier 1997) have argued that, when translating generic sentences in natural language, it is essential that we be allowed to nest default conditionals. For example, consider the following English sentences:

Close friends are (normally) people who (normally) trust one another.

People who (normally) rise early (normally) go to bed early.

In the first case, a conditional is nested within the consequent of another conditional:

∀

x∀y(Friend(x,y) ⇒ ∀z(Time(z) ⇒Trust(x,y,z)))

In the second case, we seem to have conditionals nested within both the antecedent and the consequent of a third conditional, something like:

∀

x(Person(x) → (∀y(Day(y) ⇒Rise-early(x,y)) ⇒ ∀z(Day(z) ⇒Bed-early(x,z))

This nesting of conditionals can be made possible by borrowing and modifying the semantics of the subjunctive or counterfactual conditional, developed by Robert Stalnaker and David K. Lewis (Lewis 1973). For an axiomatization of Lewis's conditional logic, see the following supplementary document:

David Lewis's Conditional Logic

The only modification that is essential is to drop the condition of
Centering (both strong and weak), a condition that makes modus ponens
(affirming the antecedent) logically valid. If the conditional ⇒
is to represent a default conditional, we do not want modus ponens to
be valid: we do not want (*p* ⇒ *q*) and *p*
to entail *q* classically (i.e., monotonically). If Centering
is dropped, the resulting logic can be made to correspond exactly to
either a preferential or a rational defeasible entailment
relation. For example, the condition of Rational Monotony is the exact
counterpart of the CV axiom of Lewis's logic:

CV: (p⇒q) → [((p&r) ⇒q) (p⇒ ¬r)]

Something like this was proposed first by James Delgrande (Delgrande
1987), and the idea has been most thoroughly developed by Nicholas
Asher and his collaborators (Asher and Morreau 1991; Asher 1995; Asher
and Bonevac 1996; Asher and Mao 2001) under the name *Commonsense
Entailment*.^{[7]}
Commonsense Entailment is a preferential
(although not a rational) consequence relation, and it automatically
satisfies the Specificity principle. It permits the arbitrary nesting
of default conditionals within other logical operators, and it can be
used to represent undercutting defeaters, through the use of negated
defaults (Asher and Mao 2001).

The models of Commonsense Entailment differ significantly from those
of preferential logic and the logic of extreme probabilities. Instead
of having structures that contain sets of *models* of a
standard, default-free language, a model the language of Commonsense
Entailment includes a set of *possible worlds*, together with a
function that assigns standard interpretation (a model of the
default-free language) to each world. In addition, to each pair
consisting of a world *w* and a set of worlds (proposition)
*A*, there is a function * that assigns a set of worlds
*(*w*,*A*) to the pair. The set
*(*w*,*A*) is the set of most normal *A*-worlds,
from the perspective of *w*. A default conditional (*p*
⇒ *q*) is true in a world *w* (in such a model)
just in case all of the most normal *p* worlds (from
*w*'s perspective) are worlds in which *q* is also
true. Since we can assign truth-conditions to each such conditional,
we can define the truth of nested conditionals, whether the
conditionals are nested within Boolean operators or within other
conditionals. Moreover, we can define both a classical, monotonic
consequence relation for this class of models and a defeasible,
nonmonotonic relation (in fact, the nonmonotonic consequence relation
can be defined in a variety of ways). We can then distinguish between
a default conditional's following *with logical necessity* from
a default theory and its following *defeasibly* from that same
theory. Contraposition, for example — inferring (¬*q*
⇒ ¬*p*) from (*p* ⇒ *q*) — is not
logically valid for default conditionals, but it might be a defeasibly
correct
inference.^{[8]}

The one critical drawback to Commonsense Entailment, when compared to the logic of extreme probabilities, is that it lacks a single, clear standard of normativity. The truth-conditions of the default conditional and the definition of nonmonotonic consequence can be fine-tuned to match many of our intuitions, but in the end of the day, the theory of Commonsense Entailment offers no simple answer to the question of what its conditional or its consequence relation are supposed (ideally) to represent.

Logics of extreme probability (beginning with the work of Ernest
Adams) did not permit the nesting of default conditionals for this
reason: the conditionals were supposed to represent something like
subjective conditional probabilities of the agent, to which the agent
was supposed to have perfect introspective access. Consequently, it
made no sense to nest this conditionals within disjunctions (as though
the agent couldn't tell which disjunct represented his actual
probability assignment) or within other conditionals (since the
subjective probability of a subjective probability is always trivial
— either exactly 1 or exactly 0). However, there is no reason
why the logic of extreme probabilities couldn't be given a different
interpretation, with (*p* ⇒ *q*) representing
something like *the objective probability of q, conditional on
p, is infinitely close to 1*. In this case, it makes
perfect sense to nest such statements of objective conditional
probability within Boolean operators (either the probability of

*q*on

*p*is close to 1, or the probability of

*r*on

*s*is close to 1), or within operators of objective probability (the objective probability that the objective probability of

*p*is close to 1 is itself close to 1). What is required in the latter case is a theory of

*higher-order probabilities*.

Fortunately, such a theory of higher-order probabilities is available (see Skyrms 1980; Gaifman 1988). The central principle of this theory is Miller's principle. For a description of the models of the logic of extreme, higher-order probability, see the following supplementary document:

Models of Higher-Order Probability

The following proposition is logically valid in this logic, representing the presence of a defeasible modus ponens rule:

((p& (p⇒q)) ⇒q)

This system can be the basis for a family of rational nonmonotonic consequence relations that include the Adams ε-entailment system as a proper part (see Koons 2000, 298-319).

### 5.9 Objections to Nonmonotonic Logic

#### Confusing Logic and Epistemology?

In an early paper (Israel 1980), David Israel raised a number of
objections to the very idea of *nonmonotonic logic*. First, he
pointed out that the nonmonotonic consequences of a finite theory are
typically not semi-decidable (recursively enumerable). This remains
true of most current systems, but it is also true of second-order
logic, infinitary logic, and a number of other systems that are now
accepted as logical in nature.

Secondly, and more to the point, Israel argued that the concept of
*nonmonotonic logic* evinces a confusion between the rules of
logic and rules of inference. In other words, Israel accused defenders
of nonmonotonic logic of confusing a theory of defeasible inference (a
branch of epistemology) with a theory of genuine consequence relations
(a branch of logic). Inference is nonmonotonic, but logic (according to
Israel) is essentially monotonic.

The best response to Israel is to point out that, like deductive logic, a theory of nonmonotonic or defeasible consequence has a number of applications besides that of guiding actual inference. Defeasible logic can be used as part of a theory of scientific explanation, and it can be used in hypothetical reasoning, as in planning. It can be used to interpret implicit features of stories, even fantastic ones, so long as it is clear which actual default rules to suspend. Thus, defeasible logic extends far beyond the boundaries of the theory of epistemic justification. Moreover, as we have seen, nonmonotonic consequence relations (especially the preferential ones) share a number of very significant formal properties with classical consequence, warranting the inclusion of them all in a larger family of logics. From this perspective, classical deductive logic is simply a special case: the study of indefeasible consequence.

#### Problems with the Deduction Theorem

In a recent paper, Charles Morgan (Morgan 2000) has argued that nonmonotonic logic is impossible. Morgan offers a series of impossibility proofs. All of Morgan's proofs turn on the fact that nonmonotonic logics cannot support a generalized deduction theorem, i.e., something of the following form:

Γ ∪ {

p}qiff Γ (p⇒q)

Morgan is certainly right about this.

However, there are good grounds for thinking that a system of
nonmonotonic logic *should* fail to include a generalized
deduction theorem. The very nature of defeasible consequence ensures
that it must be so. Consider, for example, the left-to-right direction:
suppose that Γ ∪ {*p*}
*q*. Should it follow that Γ
(*p* ⇒ *q*)?
Not at all. It may be that, normally, if *p* then
¬*q*, but Γ may contain defaults and information that
defeat and override this inference. For instance, it might contain the
fact *r* and the default ((*r* & *p*) ⇒
*q*). Similarly, consider the right-to-left direction: suppose
that Γ
(*p* ⇒ *q*). Should it follow that Γ ∪
{*p*}
*q*? Again, clearly not. Γ might contain both
*r* and a default ((*p* & *r*) ⇒
¬*q*), in which case Γ ∪ {*p*}
¬*q*.

It would be reasonable, however, to demand that a system of
nonmonotonic logic satisfy the following *special deduction
theorem*:

{

p}qiff (p⇒q)

This is certainly possible. The special deduction theorem holds
trivially, if we define{*p*}
*q* as
(*p* ⇒ *q*), that is, {*p*} defeasibly
entails *q* if and only if (by definition) (*p* ⇒
*q*) is a theorem of the classical conditional
logic.^{[9]}

## 6. Causation and Defeasible Reasoning

### 6.1 The Need for Explicit Causal Information

Hanks and McDermott, computer scientists at Yale, demonstrated that
the existing systems of nonmonotonic logic were unable to give the
right solution to a simple problem about predicting the course of
events (Hanks and McDermott 1987). The problem became known as *the
Yale shooting problem*. Hanks and McDermott assume that some sort
of *law of inertia* can be assumed: that normally properties of
things do not change. In the Yale shooting problem, there are two
relevant properties: being loaded (a property of a gun) and being
alive (a property of the intended victim of the shooting). Let's
assume that in the initial situation, *s*_{0}, the gun
is loaded and the victim is alive,
*Loaded*(*s*_{0}) and
*Alive*(*s*_{0}), and that two actions are
performed in sequence: *Wait* and *Shoot*. Let's call
the the situation that results from a moment of waiting
*s*_{1}, and the situation that follows both waiting
and then shooting *s*_{2}. There are then three
instances of the law of inertia that are relevant:

*Alive*(*s*_{0}) ⇒*Alive*(*s*_{1})*Loaded*(*s*_{0}) ⇒*Loaded*(*s*_{1})*Alive*(*s*_{1}) ⇒*Alive*(*s*_{2})

We need to make one final assumption: that shooting the victim with a loaded gun results in death (not being alive):

- ((
*Alive*(*s*_{1}) &*Loaded*(*s*_{1})) → ¬*Alive*(*s*_{2})

Intuitively, we should be able to derive the defeasible conclusion
that the victim is still alive after waiting, but dead after waiting
and shooting: *Alive*(*s*_{1}) &
¬*Alive*(*s*_{2}). However, none of the
nonmonotonic logics described above give us this result, since each of
the three instances of the law of inertia can be violated: by the
victim's inexplicably dying while we are waiting, by the gun's
miraculously becoming unloaded while we are waiting, or by the
victim's dying as a result of the shooting. Nothing introduced into
nonmonotonic logic up to this point provides us with a basis for
preferring the second exception to the law of inertia to the first or
third. What's missing is a recognition of the importance of causal
structure to defeasible
consequence.^{[10]}

There are several even simpler examples that illustrate the need to
include explicitly causal information in the input to defeasible
reasoning. Consider, for instance, this problem of Judea Pearl's (Pearl
1988): if the sprinkler is on, then normally the sidewalk is wet, and,
if the sidewalk is wet, then normally it is raining. However, we should
not infer that it is raining from the fact that the sprinkler is on.
(See Lifschitz 1990 and Lin and Reiter 1994 for additional examples of
this kind.) Similarly, if we also know that if the sidewalk is wet,
then it is slippery, we should be able to infer that the sidewalk is
slippery if the sprinkler is on and it is *not* raining.

### 6.2 Causally-Grounded Independence Relations

Hans Reichenbach, in his analysis of the interaction of causality and
probability (Reichenbach 1956), observed that the immediate causes of
an event probabilistically *screen off* from that event any
other event that is not causally posterior to it. This means that,
given the immediate causal antecedents of an event, the occurrence of
that event is rendered probabilistically independent of any
information about non-posterior events. When this insight is applied
to the nonmonotonic logic of extreme probabilities, we can use causal
information to identify which defaults function independently of
others: that is, we can decide when the fact that one default
conditional has an exception is irrelevant to the question of whether
a second conditional is also violated (see Koons 2000, 320-323). In
effect, we have a selective version of Independence of Defaults that
is grounded in causal information, enabling us to dissolve the
Drowning Problem.

For example, in the case of Pearl's sprinkler, since rain is causally
prior to the sidewalk's being wet, the causal structure of the
situation does not ensure that the rain is probabilistically
independent of whether the sprinkler is on, given the fact that the
sidewalk is wet. That is, we have no grounds for thinking that the
probability of rain, conditional on the sidewalk's being wet, is
identical to the probability of rain, conditional on the sidewalk's
being wet and the sprinkler's being on (presumably, the former is
higher than the latter). This failure of independence prevents us from
using the (*Wet* ⇒ *Rain*) default, in the presence
of the additional fact that the sprinkler is on.

In the case of the Yale shooting problem, the state of the gun's
being loaded in the aftermath of waiting,
*Loaded*(*s*_{1}), has at its only causal
antecedent the fact that the gun is loaded in
*s*_{0}. The fact of
*Loaded*(*s*_{0}) screens off the fact that the
victim is alive in *s*_{0} from the conclusion
*Loaded*(*s*_{1}). Similarly, the fact that the
victim is alive in *s*_{0} screens of the fact that the
gun is loaded in *s*_{0} from the conclusion that the
victim is still alive in *s*_{1}. In contrast, the fact
that the victim is alive at *s*_{1} does *not*
screen off the fact that the gun is loaded at *s*_{1}
from the conclusion that the victim is still alive at
*s*_{2}. Thus, we can assign higher priority to the law
of inertia with respect to both *Load* and *Alive* at
*s*_{0}, and we can conclude that the victim is alive
and the gun is loaded at *s*_{1}. The causal law for
shooting then gives us the desired conclusion, namely, that the victim
is dead at *s*_{2}.

### 6.3 Causal Circumscription

Our knowledge of causal relatedness is itself very partial. In
particular, it is difficult for us to verify conclusively that any two
randomly selected facts are or are not causally related. It seems that
in practice we apply something like Occam's razor, assuming that two
randomly selected facts are not causally related unless we have
positive reason for thinking otherwise. This invites the use of
something like circumscription, minimizing the extension of the
predicate *causes*. Once we have a set of tentative conclusions
about the causal structure of the world, we can use Reichenbach's
insight to enable us to determine which default rules should be
rendered independent of exceptions to other default rules. Since
circumscription is itself a nonmonotonic logical system, there are at
least two independent sources of nonmonotonicity or defeasibility: the
minimization or circumscription of causal relevance, and the
application of defeasible causal laws and laws of inertia.

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## Other Internet Resources

- On-line papers, Cognitive Systems Laboratory, UCLA Computer Science Department
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