Defeasible reasoning
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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 reasoning because its conclusions reach beyond the pure meanings of the premises.
Defeasible reasoning finds its fullest expression in
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, or to 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
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 (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 (from combinatorics and indifference): if p then (probably) q
- Statistical (from data and presumption): the frequency of qs among ps 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 (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
There are several notable exceptions.
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,
At the same time (in the mid-1960s), two more students of Hart and Austin at Oxford, Brian Barry and David Gauthier, were applying defeasible reasoning to political argument and practical reasoning (of action), respectively. Joel Feinberg and Joseph Raz 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'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 became the keepers of argument and rhetoric while insisting that formalism was anathema to argument.
About this time, researchers in
There are related (and slightly competing) systems of reasoning that are newer than systems of defeasible reasoning, e.g., belief revision 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 that were inalienable, divine, or indubitable.
For instance, the 1776
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.[citation needed]
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,
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 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.,
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), 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 systemas 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 preferentialsemantics 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
- 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 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
- Argument – Attempt to persuade or to determine the truth of a conclusion
- Artificial intelligence and law– Information science by discipline
- Defeasible estate – Estate created when land is transferred conditionally
- Hasty generalization– Conclusion made on the basis of one or few instances of a phenomenon
- Indefeasible rights of use – long-term lease of bandwidth
- Practical reason – Use of reason to decide how to act
- Defeater
References
- ^ ""Defeasible Reasoning," Stanford Encyclopedia of Philosophy". Retrieved 1 July 2016.
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, eds., Kluwer, 2002.
- Dialectics, Nicholas Rescher, 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, Proc. 5th Intl. Conf. on AI and Law, 1995.
- Political argument, Brian Barry, Routledge & Kegan Paul, 1970.
- The uses of argument, Stephen Toulmin, 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.
- Lombrozo, Tania (2011). "Defeasibility". Edge. Retrieved 6 June 2016.
- Hage, Jaap (2003). "Law and defeasibility" (PDF). Artificial Intelligence and Law. 11 (2/3): 221–243. S2CID 12271954. Retrieved 6 June 2016.