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In
philosophical logic Understood in a narrow sense, philosophical logic is the area of logic that studies the application of logical methods to philosophical problems, often in the form of extended logical systems like modal logic. Some theorists conceive philosophical ...
, defeasible reasoning is a kind of
reasoning Reason is the capacity of consciously applying logic by drawing conclusions from new or existing information, with the aim of seeking the truth. It is closely associated with such characteristically human activities as philosophy, science, langu ...
that is rationally compelling, though not deductively valid. It usually occurs when a rule is given, but there may be specific exceptions to the rule, or subclasses that are subject to a different rule. Defeasibility is found in literatures that are concerned with
argument An argument is a statement or group of statements called premises intended to determine the degree of truth or acceptability of another statement called conclusion. Arguments can be studied from three main perspectives: the logical, the dialecti ...
and the process of argument, or
heuristic A heuristic (; ), or heuristic technique, is any approach to problem solving or self-discovery that employs a practical method that is not guaranteed to be optimal, perfect, or rational, but is nevertheless sufficient for reaching an immediate ...
reasoning. Defeasible reasoning is a particular kind of non-demonstrative reasoning, where the reasoning does not produce a full, complete, or final demonstration of a claim, i.e., where fallibility and corrigibility of a conclusion are acknowledged. In other words, defeasible reasoning produces a contingent statement or claim. Defeasible reasoning is also a kind of
ampliative Ampliative (from Latin ''ampliare'', "to enlarge"), a term used mainly in logic, meaning "extending" or "adding to that which is already known". This terminology was often used by medieval logicians in the analyses of the temporal content of thei ...
reasoning because its conclusions reach beyond the pure meanings of the premises. Defeasible reasoning finds its fullest expression in
jurisprudence Jurisprudence, or legal theory, is the theoretical study of the propriety of law. Scholars of jurisprudence seek to explain the nature of law in its most general form and they also seek to achieve a deeper understanding of legal reasoning ...
,
ethics Ethics or moral philosophy is a branch of philosophy that "involves systematizing, defending, and recommending concepts of right and wrong behavior".''Internet Encyclopedia of Philosophy'' The field of ethics, along with aesthetics, concer ...
and
moral philosophy Ethics or moral philosophy is a branch of philosophy that "involves systematizing, defending, and recommending concepts of right and wrong behavior".''Internet Encyclopedia of Philosophy'' The field of ethics, along with aesthetics, concerns ...
,
epistemology Epistemology (; ), or the theory of knowledge, is the branch of philosophy concerned with knowledge. Epistemology is considered a major subfield of philosophy, along with other major subfields such as ethics, logic, and metaphysics. Epi ...
,
pragmatics In linguistics and related fields, pragmatics is the study of how context contributes to meaning. The field of study evaluates how human language is utilized in social interactions, as well as the relationship between the interpreter and the int ...
and conversational conventions in
linguistics Linguistics is the scientific study of human language. It is called a scientific study because it entails a comprehensive, systematic, objective, and precise analysis of all aspects of language, particularly its nature and structure. Ling ...
, constructivist decision theories, and in
knowledge representation Knowledge representation and reasoning (KRR, KR&R, KR²) is the field of artificial intelligence (AI) dedicated to representing information about the world in a form that a computer system can use to solve complex tasks such as diagnosing a medic ...
and
planning Planning is the process of thinking regarding the activities required to achieve a desired goal. Planning is based on foresight, the fundamental capacity for mental time travel. The evolution of forethought, the capacity to think ahead, is c ...
in
artificial intelligence Artificial intelligence (AI) is intelligence—perceiving, synthesizing, and inferring information—demonstrated by machines, as opposed to intelligence displayed by animals and humans. Example tasks in which this is done include speech ...
. It is also closely identified with
prima facie ''Prima facie'' (; ) is a Latin expression meaning ''at first sight'' or ''based on first impression''. The literal translation would be 'at first face' or 'at first appearance', from the feminine forms of ''primus'' ('first') and ''facies'' (' ...
(presumptive) reasoning (i.e., reasoning on the "face" of evidence), and
ceteris paribus ' (also spelled '; () is a Latin phrase, meaning "other things equal"; some other English translations of the phrase are "all other things being equal", "other things held constant", "all else unchanged", and "all else being equal". A statement ...
(default) reasoning (i.e., reasoning, all things "being equal"). According to at least some schools of philosophy, ''all'' reasoning is at most defeasible, and there is no such thing as absolutely certain deductive reasoning, since it is impossible to be absolutely certain of all the facts (and know with certainty that nothing is unknown). Thus all deductive reasoning is in reality contingent and defeasible.


Other kinds of non-demonstrative reasoning

Other kinds of non-demonstrative reasoning are
probabilistic reasoning Probabilistic logic (also probability logic and probabilistic reasoning) involves the use of probability and logic to deal with uncertain situations. Probabilistic logic extends traditional logic truth tables with probabilistic expressions. A diffic ...
,
inductive reasoning Inductive reasoning is a method of reasoning in which a general principle is derived from a body of observations. It consists of making broad generalizations based on specific observations. Inductive reasoning is distinct from ''deductive'' re ...
,
statistical Statistics (from German: '' Statistik'', "description of a state, a country") is the discipline that concerns the collection, organization, analysis, interpretation, and presentation of data. In applying statistics to a scientific, industr ...
reasoning,
abductive reasoning Abductive reasoning (also called abduction,For example: abductive inference, or retroduction) is a form of logical inference formulated and advanced by American philosopher Charles Sanders Peirce beginning in the last third of the 19th century ...
, and
paraconsistent A paraconsistent logic is an attempt at a logical system to deal with contradictions in a discriminating way. Alternatively, paraconsistent logic is the subfield of logic that is concerned with studying and developing "inconsistency-tolerant" syste ...
reasoning. The differences between these kinds of reasoning correspond to differences about the conditional that each kind of reasoning uses, and on what premise (or on what authority) the conditional is adopted: * ''
Deductive Deductive reasoning is the mental process of drawing deductive inferences. An inference is deductively valid if its conclusion follows logically from its premises, i.e. if it is impossible for the premises to be true and the conclusion to be fals ...
'' (from meaning postulate or axiom): if ''p'' then ''q'' (equivalent to ''q'' or ''not-p'' in classical logic, not necessarily in other logics) * ''Defeasible'' (from authority): if ''p'' then (defeasibly) ''q'' * ''
Probabilistic Probability is the branch of mathematics concerning numerical descriptions of how likely an event is to occur, or how likely it is that a proposition is true. The probability of an event is a number between 0 and 1, where, roughly speaking, ...
'' (from combinatorics and indifference): if ''p'' then (probably) ''q'' * ''
Statistical Statistics (from German: '' Statistik'', "description of a state, a country") is the discipline that concerns the collection, organization, analysis, interpretation, and presentation of data. In applying statistics to a scientific, industr ...
'' (from data and presumption): the frequency of ''q''s among ''p''s is high (or inference from a model fit to data); hence, (in the right context) if ''p'' then (probably) ''q'' * '' Inductive'' (theory formation; from data, coherence, simplicity, and confirmation): (inducibly) "if ''p'' then ''q''"; hence, if ''p'' then (deducibly-but-revisably) ''q'' * ''
Abductive Abductive reasoning (also called abduction,For example: abductive inference, or retroduction) is a form of logical inference formulated and advanced by American philosopher Charles Sanders Peirce beginning in the last third of the 19th centur ...
'' (from data and theory): ''p'' and ''q'' are correlated, and ''q'' is sufficient for ''p''; hence, if ''p'' then (abducibly) ''q'' as cause


History

Though
Aristotle Aristotle (; grc-gre, Ἀριστοτέλης ''Aristotélēs'', ; 384–322 BC) was a Greek philosopher and polymath during the Classical period in Ancient Greece. Taught by Plato, he was the founder of the Peripatetic school of ...
differentiated the forms of reasoning that are valid for
logic Logic is the study of correct reasoning. It includes both formal and informal logic. Formal logic is the science of deductively valid inferences or of logical truths. It is a formal science investigating how conclusions follow from prem ...
and
philosophy Philosophy (from , ) is the systematized study of general and fundamental questions, such as those about existence, reason, knowledge, values, mind, and language. Such questions are often posed as problems to be studied or resolved. ...
from the more general ones that are used in everyday life (see
dialectics Dialectic ( grc-gre, διαλεκτική, ''dialektikḗ''; related to dialogue; german: Dialektik), also known as the dialectical method, is a discourse between two or more people holding different points of view about a subject but wishing t ...
and
rhetoric Rhetoric () is the art of persuasion, which along with grammar and logic (or dialectic), is one of the three ancient arts of discourse. Rhetoric aims to study the techniques writers or speakers utilize to inform, persuade, or motivate par ...
), 20th century philosophers mainly concentrated on deductive reasoning. At the end of the 19th century, logic texts would typically survey both demonstrative and non-demonstrative reasoning, often giving more space to the latter. However, after the blossoming of
mathematical logic Mathematical logic is the study of formal logic within mathematics. Major subareas include model theory, proof theory, set theory, and recursion theory. Research in mathematical logic commonly addresses the mathematical properties of forma ...
at the hands of
Bertrand Russell Bertrand Arthur William Russell, 3rd Earl Russell, (18 May 1872 – 2 February 1970) was a British mathematician, philosopher, logician, and public intellectual. He had a considerable influence on mathematics, logic, set theory, linguistics, a ...
,
Alfred North Whitehead Alfred North Whitehead (15 February 1861 – 30 December 1947) was an English mathematician and philosopher. He is best known as the defining figure of the philosophical school known as process philosophy, which today has found applica ...
and
Willard van Orman Quine Willard Van Orman Quine (; known to his friends as "Van"; June 25, 1908 – December 25, 2000) was an American philosopher and logician in the analytic tradition, recognized as "one of the most influential philosophers of the twentieth century" ...
, latter-20th century logic texts paid little attention to the non-deductive modes of inference. There are several notable exceptions.
John Maynard Keynes John Maynard Keynes, 1st Baron Keynes, ( ; 5 June 1883 – 21 April 1946), was an English economist whose ideas fundamentally changed the theory and practice of macroeconomics and the economic policies of governments. Originally trained in ...
wrote his dissertation on non-demonstrative reasoning, and influenced the thinking of
Ludwig Wittgenstein Ludwig Josef Johann Wittgenstein ( ; ; 26 April 1889 – 29 April 1951) was an Austrian- British philosopher who worked primarily in logic, the philosophy of mathematics, the philosophy of mind, and the philosophy of language. He is consi ...
on this subject. Wittgenstein, in turn, had many admirers, including the positivist legal scholar H.L.A. Hart and the
speech act In the philosophy of language and linguistics, speech act is something expressed by an individual that not only presents information but performs an action as well. For example, the phrase "I would like the kimchi; could you please pass it to me? ...
linguist
John L. Austin John Langshaw Austin (26 March 1911 – 8 February 1960) was a British people, British philosophy of language, philosopher of language and leading proponent of ordinary language philosophy, perhaps best known for developing the theory of speech ...
,
Stephen Toulmin Stephen Edelston Toulmin (; 25 March 1922 – 4 December 2009) was a British philosopher, author, and educator. Influenced by Ludwig Wittgenstein, Toulmin devoted his works to the analysis of moral reasoning. Throughout his writings, he sought ...
and
Chaim Perelman The name ''Haim'' can be a first name or surname originating in the Hebrew language, or deriving from the Old German name '' Haimo''. Hebrew etymology Chayyim ( he, חַיִּים ', Classical Hebrew: , Israeli Hebrew: ), also transcribed ''Ha ...
in rhetoric, the moral theorists
W.D. Ross WD may refer to: Arts and entertainment * ''The Walking Dead'' (TV series) * ''White Dwarf'' (magazine) Businesses and organizations Government agencies * Royal Canadian Air Force Women's Division * War Department (United Kingdom) Oth ...
and C.L. Stevenson, and the
vagueness In linguistics and philosophy, a vague predicate is one which gives rise to borderline cases. For example, the English adjective "tall" is vague since it is not clearly true or false for someone of middling height. By contrast, the word "prime" is ...
epistemologist/ontologist Friedrich Waismann. The etymology of ''defeasible'' usually refers to Middle English law of contracts, where a condition of defeasance is a clause that can invalidate or annul a contract or deed. Though ''defeat'', ''dominate'', ''defer'', ''defy'', ''deprecate'' and ''derogate'' are often used in the same contexts as ''defease,'' the verbs ''annul'' and ''invalidate'' (and ''nullify,'' ''overturn,'' ''rescind,'' ''vacate,'' ''repeal,'' ''void'', ''cancel'', ''countermand'', ''preempt'', etc.) are more properly correlated with the concept of defeasibility than those words beginning with the letter ''d''. Many dictionaries do contain the verb, ''to defease'' with past participle, ''defeased.'' Philosophers in moral theory and rhetoric had taken defeasibility largely for granted when American epistemologists rediscovered Wittgenstein's thinking on the subject: John Ladd,
Roderick Chisholm Roderick Milton Chisholm (; November 27, 1916 – January 19, 1999) was an American philosopher known for his work on epistemology, metaphysics, free will, value theory, and the philosophy of perception. The '' Stanford Encyclopedia of Phi ...
,
Roderick Firth Roderick Firth (January 30, 1917 – December 22, 1987) was an American philosopher. He was Professor of Philosophy at Harvard University from 1953 until his death. Education Firth earned his Ph.D. in philosophy from Harvard in 1943. His thesis ...
,
Ernest Sosa Ernest Sosa (born June 17, 1940) is an American philosopher primarily interested in epistemology. Since 2007 he has been Board of Governors Professor of Philosophy at Rutgers University, but he spent most of his career at Brown University. Educa ...
,
Robert Nozick Robert Nozick (; November 16, 1938 – January 23, 2002) was an American philosopher. He held the Joseph Pellegrino University Professorship at Harvard University,
, and
John L. Pollock John L. Pollock (1940–2009) was an American philosopher known for influential work in epistemology, philosophical logic, cognitive science, and artificial intelligence. Life and career Born John Leslie Pollock in Atchison, Kansas, on January ...
all began writing with new conviction about how ''appearance as red'' was only a defeasible reason for believing something to be red. More importantly Wittgenstein's orientation toward language-games (and away from
semantics Semantics (from grc, σημαντικός ''sēmantikós'', "significant") is the study of reference, meaning, or truth. The term can be used to refer to subfields of several distinct disciplines, including philosophy, linguistics and comput ...
) emboldened these epistemologists to manage rather than to expurgate ''prima facie'' logical inconsistency. At the same time (in the mid-1960s), two more students of Hart and Austin at Oxford,
Brian Barry Brian Barry, (7 August 1936 – 10 March 2009) was a moral and political philosopher. He was educated at the Queen's College, Oxford, obtaining the degrees of B.A. and D.Phil. under the direction of H. L. A. Hart. Along with David Braybrooke ...
and
David Gauthier David Gauthier (; born 10 September 1932) is a Canadian-American philosopher best known for his neo-Hobbesian social contract (contractarian) theory of morality, as developed in his 1986 book ''Morals by Agreement''. Life and career Gauthie ...
, were applying defeasible reasoning to political argument and practical reasoning (of action), respectively.
Joel Feinberg Joel Feinberg (October 19, 1926 in Detroit, Michigan – March 29, 2004 in Tucson, Arizona) was an American political and legal philosopher. He is known for his work in the fields of ethics, action theory, philosophy of law, and political p ...
and
Joseph Raz Joseph Raz (; he, יוסף רז; born Zaltsman; 21 March 19392 May 2022) was an Israeli legal, moral and political philosopher. He was an advocate of legal positivism and is known for his conception of perfectionist liberalism. Raz spent mos ...
were beginning to produce equally mature works in ethics and jurisprudence informed by defeasibility. By far the most significant works on defeasibility by the mid-1970s were in epistemology, where John Pollock's 1974 ''Knowledge and Justification'' popularized his terminology of ''undercutting'' and ''rebutting'' (which mirrored the analysis of Toulmin). Pollock's work was significant precisely because it brought defeasibility so close to philosophical logicians. The failure of logicians to dismiss defeasibility in epistemology (as Cambridge's logicians had done to Hart decades earlier) landed defeasible reasoning in the philosophical mainstream. Defeasibility had always been closely related to argument, rhetoric, and law, except in epistemology, where the chains of reasons, and the origin of reasons, were not often discussed.
Nicholas Rescher Nicholas Rescher (; ; born 15 July 1928) is a German-American philosopher, polymath, and author, who has been a professor of philosophy at the University of Pittsburgh since 1961. He is chairman of the Center for Philosophy of Science and was fo ...
's ''Dialectics'' is an example of how difficult it was for philosophers to contemplate more complex systems of defeasible reasoning. This was in part because proponents of
informal logic Informal logic encompasses the principles of logic and logical thought outside of a formal setting (characterized by the usage of particular statements). However, the precise definition of "informal logic" is a matter of some dispute. Ralph H. ...
became the keepers of argument and rhetoric while insisting that formalism was anathema to argument. About this time, researchers in
artificial intelligence Artificial intelligence (AI) is intelligence—perceiving, synthesizing, and inferring information—demonstrated by machines, as opposed to intelligence displayed by animals and humans. Example tasks in which this is done include speech ...
became interested in
non-monotonic reasoning A non-monotonic logic is a formal logic whose conclusion relation is not monotonic. In other words, non-monotonic logics are devised to capture and represent defeasible inferences (cf. defeasible reasoning), i.e., a kind of inference in which re ...
and its
semantics Semantics (from grc, σημαντικός ''sēmantikós'', "significant") is the study of reference, meaning, or truth. The term can be used to refer to subfields of several distinct disciplines, including philosophy, linguistics and comput ...
. With philosophers such as Pollock and Donald Nute (e.g.,
defeasible logic Defeasible logic is a non-monotonic logic proposed by Donald Nute to formalize defeasible reasoning. In defeasible logic, there are three different types of propositions: ; strict rules : specify that a fact is always a consequence of another; ; ...
), dozens of computer scientists and logicians produced complex systems of defeasible reasoning between 1980 and 2000. No single system of defeasible reasoning would emerge in the same way that Quine's system of logic became a de facto standard. Nevertheless, the 100-year headstart on non-demonstrative logical calculi, due to
George Boole George Boole (; 2 November 1815 – 8 December 1864) was a largely self-taught English mathematician, philosopher, and logician, most of whose short career was spent as the first professor of mathematics at Queen's College, Cork in ...
,
Charles Sanders Peirce Charles Sanders Peirce ( ; September 10, 1839 – April 19, 1914) was an American philosopher, logician, mathematician and scientist who is sometimes known as "the father of pragmatism". Educated as a chemist and employed as a scientist for ...
, and
Gottlob Frege Friedrich Ludwig Gottlob Frege (; ; 8 November 1848 – 26 July 1925) was a German philosopher, logician, and mathematician. He was a mathematics professor at the University of Jena, and is understood by many to be the father of analytic p ...
was being closed: both demonstrative and non-demonstrative reasoning now have formal calculi. There are related (and slightly competing) systems of reasoning that are newer than systems of defeasible reasoning, e.g.,
belief revision Belief revision is the process of changing beliefs to take into account a new piece of information. The logical formalization of belief revision is researched in philosophy, in databases, and in artificial intelligence for the design of rational ag ...
and dynamic logic. The dialogue logics of Charles Hamblin and Jim Mackenzie, and their colleagues, can also be tied closely to defeasible reasoning. Belief revision is a non-constructive specification of the desiderata with which, or constraints according to which, epistemic change takes place. Dynamic logic is related mainly because, like paraconsistent logic, the reordering of premises can change the set of justified conclusions. Dialogue logics introduce an adversary, but are like belief revision theories in their adherence to deductively consistent states of belief.


Political and judicial use

Many political philosophers have been fond of the word ''indefeasible'' when referring to rights, e.g., that were ''inalienable,'' ''divine,'' or ''indubitable.'' For example, in the 1776
Virginia Declaration of Rights The Virginia Declaration of Rights was drafted in 1776 to proclaim the inherent rights of men, including the right to reform or abolish "inadequate" government. It influenced a number of later documents, including the United States Declaratio ...
, "community hath an indubitable, inalienable, and indefeasible right to reform, alter or abolish government..." (also attributed to
James Madison James Madison Jr. (March 16, 1751June 28, 1836) was an American statesman, diplomat, and Founding Father. He served as the fourth president of the United States from 1809 to 1817. Madison is hailed as the "Father of the Constitution" for h ...
); and
John Adams John Adams (October 30, 1735 – July 4, 1826) was an American statesman, attorney, diplomat, writer, and Founding Fathers of the United States, Founding Father who served as the second president of the United States from 1797 to 1801. Befor ...
, "The people have a right, an indisputable, unalienable, indefeasible, divine right to that most dreaded and envied kind of knowledge – I mean of the character and conduct of their rulers." Also,
Lord Aberdeen George Hamilton-Gordon, 4th Earl of Aberdeen, (28 January 178414 December 1860), styled Lord Haddo from 1791 to 1801, was a British statesman, diplomat and landowner, successively a Tory, Conservative and Peelite politician and specialist in ...
: "indefeasible right inherent in the British Crown" and
Gouverneur Morris Gouverneur Morris ( ; January 31, 1752 – November 6, 1816) was an American statesman, a Founding Father of the United States, and a signatory to the Articles of Confederation and the United States Constitution. He wrote the Preamble to th ...
: "the Basis of our own Constitution is the indefeasible Right of the People." Scholarship about
Abraham Lincoln Abraham Lincoln ( ; February 12, 1809 – April 15, 1865) was an American lawyer, politician, and statesman who served as the 16th president of the United States from 1861 until his assassination in 1865. Lincoln led the nation throu ...
often cites these passages in the justification of secession. Philosophers who use the word ''defeasible'' have historically had different world views from those who use the word ''indefeasible'' (and this distinction has often been mirrored by Oxford and Cambridge zeitgeist); hence it is rare to find authors who use both words. In judicial opinions, the use of ''defeasible'' is commonplace. There is however disagreement among legal logicians whether ''defeasible reasoning'' is central, e.g., in the consideration of ''open texture'',
precedent A precedent is a principle or rule established in a previous legal case that is either binding on or persuasive for a court or other tribunal when deciding subsequent cases with similar issues or facts. Common-law legal systems place great v ...
, exceptions, and ''rationales'', or whether it applies only to explicit defeasance clauses. H.L.A. Hart in ''
The Concept of Law ''The Concept of Law'' is a 1961 book by the legal philosopher H. L. A. Hart and his most famous work. ''The Concept of Law'' presents Hart's theory of legal positivism—the view that laws are rules made by humans and that there is ...
'' gives two famous examples of defeasibility: "No vehicles in the park" (except during parades); and "Offer, acceptance, and memorandum produce a contract" (except when the contract is illegal, the parties are minors, inebriated, or incapacitated, etc.).


Specificity

One of the main disputes among those who produce systems of defeasible reasoning is the status of a ''rule of specificity.'' In its simplest form, it is the same rule as subclass
inheritance Inheritance is the practice of receiving private property, titles, debts, entitlements, privileges, rights, and obligations upon the death of an individual. The rules of inheritance differ among societies and have changed over time. Of ...
preempting class inheritance: (R1) if ''r'' then (defeasibly) ''q'' e.g., if bird, then can fly (R2) if ''p'' then (defeasibly) ''not-q'' e.g., if penguin, then cannot fly (O1) if ''p'' then (deductively) ''r'' e.g., if penguin, then bird (M1) arguably, p e.g., arguably, penguin (M2) R2 is a more specific reason than R1 e.g., R2 is better than R1 (M3) therefore, arguably, not-q e.g., therefore, arguably, cannot fly Approximately half of the systems of defeasible reasoning discussed today adopt a rule of specificity, while half expect that such ''preference'' rules be written explicitly by whoever provides the defeasible reasons. For example, Rescher's dialectical system uses specificity, as do early systems of multiple inheritance (e.g.,
David Touretzky David S. Touretzky is a research professor in the Computer Science Department and the Center for the Neural Basis of Cognition at Carnegie Mellon University. He received a BA in Computer Science at Rutgers University in 1978, and earned a master's ...
) and the early argument systems of Donald Nute and of Guillermo Simari and
Ronald Loui Ronald Prescott Loui is an American computer scientist, currently working as a professor of computer science at Case Western Reserve University. He is known for having supplied first-hand biographical information on Barack Obama about his time in H ...
. Defeasible reasoning accounts of precedent (
stare decisis A precedent is a principle or rule established in a previous legal case that is either binding on or persuasive for a court or other tribunal when deciding subsequent cases with similar issues or facts. Common-law legal systems place great va ...
and
case-based reasoning In artificial intelligence and philosophy, case-based reasoning (CBR), broadly construed, is the process of solving new problems based on the solutions of similar past problems. In everyday life, an auto mechanic who fixes an engine by recallin ...
) also make use of specificity (e.g.,
Joseph Raz Joseph Raz (; he, יוסף רז; born Zaltsman; 21 March 19392 May 2022) was an Israeli legal, moral and political philosopher. He was an advocate of legal positivism and is known for his conception of perfectionist liberalism. Raz spent mos ...
and the work of Kevin D. Ashley and Edwina Rissland). Meanwhile, the argument systems of Henry Prakken and Giovanni Sartor, of Bart Verheij and Jaap Hage, and the system of Phan Minh Dung do not adopt such a rule.


Nature of defeasibility

There is a distinct difference between those who theorize about defeasible reasoning as if it were a system of confirmational revision (with affinities to
belief revision Belief revision is the process of changing beliefs to take into account a new piece of information. The logical formalization of belief revision is researched in philosophy, in databases, and in artificial intelligence for the design of rational ag ...
), and those who theorize about defeasibility as if it were the result of further (non-empirical) investigation. There are at least three kinds of further non-empirical investigation: progress in a lexical/syntactic process, progress in a computational process, and progress in an adversary or legal proceeding. ; Defeasibility as corrigibility : Here, a person learns something new that annuls a prior inference. In this case, defeasible reasoning provides a constructive mechanism for belief revision, like a
truth maintenance system {{more footnotes, date=September 2009 Reason maintenanceDoyle, J., 1983. The ins and outs of reason maintenance, in: Proceedings of the Eighth International Joint Conference on Artificial Intelligence - Volume 1, IJCAI’83. Morgan Kaufmann Publish ...
as envisioned by Jon Doyle. ; Defeasibility as shorthand for preconditions : Here, the author of a set of rules or legislative code is writing rules with exceptions. Sometimes a set of defeasible rules can be rewritten, with more cogency, with explicit (local) pre-conditions instead of (non-local) competing rules. Many non-monotonic systems with fixed-point or preferential semantics fit this view. However, sometimes the rules govern a process of argument (the last view on this list), so that they cannot be re-compiled into a set of deductive rules lest they lose their force in situations with incomplete knowledge or incomplete derivation of preconditions. ; Defeasibility as an
anytime algorithm In computer science, an anytime algorithm is an algorithm that can return a valid solution to a problem even if it is interrupted before it ends. The algorithm is expected to find better and better solutions the longer it keeps running. Most algo ...
: Here, it is assumed that calculating arguments takes time, and at any given time, based on a subset of the potentially constructible arguments, a conclusion is defeasibly justified.
Isaac Levi Isaac Levi (June 30, 1930 – December 25, 2018) was an American philosopher who served as the John Dewey Professor of Philosophy at Columbia University. He is noted for his work in epistemology and decision theory. Education and career Levi wa ...
has protested against this kind of defeasibility, but it is well-suited to the heuristic projects of, for example, Herbert A. Simon. On this view, the ''best move so far'' in a chess-playing program's analysis at a particular depth is a defeasibly justified conclusion. This interpretation works with either the prior or the next semantical view. ; Defeasibility as a means of controlling an investigative or social process : Here, justification is the result of the right kind of procedure (e.g., a fair and efficient hearing), and defeasible reasoning provides impetus for pro and con responses to each other. Defeasibility has to do with the alternation of verdict as locutions are made and cases presented, not the changing of a mind with respect to new (empirical) discovery. Under this view, defeasible reasoning and defeasible argumentation refer to the same phenomenon.


See also

* * * * * * *
Defeater A defeater of a belief is evidence that this belief is false. Defeaters are of particular interest to epistemology since they affect whether a belief is justified. An important distinction is between undercutting and rebutting defeaters. Undercu ...


References


Further reading


Defeasible logic
Donald Nute, Lecture Notes in Computer Science, Springer, 2003.
Logical models of argument
Carlos Chesnevar, et al., ACM Computing Surveys 32:4, 2000.
Logics for defeasible argumentation
Henry Prakken and Gerard Vreeswijk, in Handbook of Philosophical Logic, Dov M. Gabbay,
Franz Guenthner Franz Guenthner is a German linguist who is a professor of Computational Linguistics at the Center for Information and Language Processing (CIS) at the Ludwig-Maximilians-Universität (LMU) in Munich, Germany. His background is in philosophy and ...
, eds., Kluwer, 2002.
Dialectics
Nicholas Rescher Nicholas Rescher (; ; born 15 July 1928) is a German-American philosopher, polymath, and author, who has been a professor of philosophy at the University of Pittsburgh since 1961. He is chairman of the Center for Philosophy of Science and was fo ...
, SUNY Press, 1977.
Defeasible reasoning
John Pollock, Cognitive Science, 1987.
Knowledge and Justification
John Pollock, Princeton University Press, 1974.
Hart's critics on defeasible concepts and ascriptivism
Ronald Loui Ronald Prescott Loui is an American computer scientist, currently working as a professor of computer science at Case Western Reserve University. He is known for having supplied first-hand biographical information on Barack Obama about his time in H ...
, Proc. 5th Intl. Conf. on AI and Law, 1995.
Political argument
Brian Barry Brian Barry, (7 August 1936 – 10 March 2009) was a moral and political philosopher. He was educated at the Queen's College, Oxford, obtaining the degrees of B.A. and D.Phil. under the direction of H. L. A. Hart. Along with David Braybrooke ...
, Routledge & Kegan Paul, 1970.
The uses of argument
Stephen Toulmin Stephen Edelston Toulmin (; 25 March 1922 – 4 December 2009) was a British philosopher, author, and educator. Influenced by Ludwig Wittgenstein, Toulmin devoted his works to the analysis of moral reasoning. Throughout his writings, he sought ...
, Cambridge University Press, 1958.
Discourse relations and defeasible knowledge
Alex Lascarides and Nicholas Asher, Proc. of the 29th Meeting of the Assn. for Comp. Ling., 1991.
Defeasible logic programming: an argumentative approach
Alejandro Garcia and Guillermo Simari, Theory and Practice of Logic Programming 4:95–138, 2004.
Philosophical foundations of deontic logic and the logic of defeasible conditionals
Carlos Alchourron, in Deontic logic in computer science: normative system specification, J. Meyer, R. Wieringa, eds., Wiley, 1994.
A Mathematical Treatment of Defeasible Reasoning and its Implementation.
Guillermo Simari,
Ronald Loui Ronald Prescott Loui is an American computer scientist, currently working as a professor of computer science at Case Western Reserve University. He is known for having supplied first-hand biographical information on Barack Obama about his time in H ...
, Artificial Intelligence, 53(2–3): 125–157 (1992). * * {{cite journal , last=Hage , first=Jaap , year=2003 , title=Law and defeasibility , url=http://www.umiacs.umd.edu/~horty/courses/readings/hage-2003-defeasibility.pdf , journal=Artificial Intelligence and Law , volume=11 , issue=2/3 , pages=221–243 , doi=10.1023/B:ARTI.0000046011.13621.08 , s2cid=12271954 , access-date=6 June 2016


External links


Article on Defeasible Reasoning
in the
Stanford Encyclopedia of Philosophy The ''Stanford Encyclopedia of Philosophy'' (''SEP'') combines an online encyclopedia of philosophy with peer-reviewed publication of original papers in philosophy, freely accessible to Internet users. It is maintained by Stanford University. E ...

An example of defeasible reasoning in action
Belief revision Epistemology Logic Logic programming Knowledge representation Reasoning