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A priori ('from the earlier') anda posteriori ('from the later') areLatin phrases used inphilosophy to distinguish types ofknowledge,justification, orargument by their reliance on experience.A priori knowledge is independent from anyexperience. Examples includemathematics,[i]tautologies anddeduction frompure reason.[ii]A posteriori knowledge depends onempirical evidence. Examples include most fields ofscience and aspects ofpersonal knowledge.
The terms originate from the analytic methods found inOrganon, a collection of works byAristotle.Prior analytics (a priori) is aboutdeductive logic, which comes from definitions and first principles.Posterior analytics (a posteriori) is aboutinductive logic, which comes from observational evidence.
Both terms appear inEuclid'sElements and were popularized byImmanuel Kant'sCritique of Pure Reason, an influential work in thehistory of philosophy.[1] Both terms are primarily used asmodifiers to thenounknowledge (e.g.,a priori knowledge).A priori can be used to modify other nouns such astruth. Philosophers may useapriority,apriorist andaprioricity as nouns referring to the quality of beinga priori.[2]
Consider theproposition: "IfGeorge V reigned at least four days, then he reigned more than three days." This is something that one knowsa priori because it expresses a statement that one can derive by reason alone.
Consider the proposition: "George V reigned from 1910 to 1936." This is something that (if true) one must come to knowa posteriori because it expresses an empirical fact unknowable by reason alone.
Several philosophers, in reaction toImmanuel Kant, sought to explaina priori knowledge without appealing to whatPaul Boghossian describes as "a special faculty [intuition] ... that has never been described in satisfactory terms."[3] One theory, popular among thelogical positivists of the early 20th century, is what Boghossian calls the "analytic explanation of the a priori".[3] The distinction betweenanalytic and synthetic propositions was first introduced by Kant. While his original distinction was primarily drawn in terms of conceptual containment, the contemporary version of such distinction primarily involves, as American philosopherWillard Van Orman Quine put it, the notions of "true by virtue of meanings and independently of fact."[4]
Analytic propositions are considered true by virtue of their meaning alone, whilea posteriori propositions by virtue of their meaning and of certain facts about the world. According to the analytic explanation of thea priori, alla priori knowledge is analytic; soa priori knowledge need not require a special faculty of pureintuition, since it can be accounted for simply by one's ability to understand the meaning of the proposition in question. More simply, proponents of this explanation claimed to have reduced a dubiousmetaphysical faculty of pure reason to a legitimate linguistic notion of analyticity.
The analytic explanation ofa priori knowledge has undergone several criticisms. Most notably, Quine argues that the analytic–synthetic distinction is illegitimate:[5]
But for all its a priori reasonableness, a boundary between analytic and synthetic statements simply has not been drawn. That there is such a distinction to be drawn at all is an unempirical dogma of empiricists, a metaphysical article of faith.
Although the soundness of Quine's proposition remains uncertain, it had a powerful effect on the project of explaining thea priori in terms of the analytic.[6]
The metaphysical distinction betweennecessary andcontingent truths has also been related toa priori anda posteriori knowledge.
A proposition that isnecessarily true is one in which its negation is self-contradictory; it is true in everypossible world. For example, considering the proposition "all bachelors are unmarried:" its negation (i.e. the proposition that some bachelors are married) is incoherent due to the concept of being unmarried (or the meaning of the word "unmarried") being tied to part of the concept of being a bachelor (or part of the definition of the word "bachelor"). To the extent that contradictions are impossible, self-contradictory propositions are necessarily false as it is impossible for them to be true. The negation of a self-contradictory proposition is, therefore, supposed to be necessarily true.
By contrast, a proposition that iscontingently true is one in which its negation is not self-contradictory. Thus, it is saidnot to be true in every possible world. As Jason Baehr suggests, it seems plausible that all necessary propositions are knowna priori, because "[s]ense experience can tell us only about the actual world and hence about what is the case; it can say nothing about what must or must not be the case."[7]
Following Kant, some philosophers have considered the relationship betweenaprioricity,analyticity andnecessity to be extremely close. According toJerry Fodor, "positivism, in particular, took it for granted thata priori truths must be necessary."[8] Since Kant, the distinction between analytic and synthetic propositions has slightly changed.Analytic propositions were largely taken to be "true by virtue of meanings and independently of fact",[4] while synthetic propositions were not—one must conduct some sort of empirical investigation, looking to the world, to determine thetruth-value of synthetic propositions.
Aprioricity, analyticity and necessity have since been more clearly separated from each other. American philosopherSaul Kripke (1972), for example, provides strong arguments against this position, whereby he contends that there are necessarya posteriori truths. For example, the proposition that water is H2O (if it is true): According to Kripke, this statement is bothnecessarily true, because water and H2O are the same thing, they are identical in every possible world, and truths of identity are logically necessary; anda posteriori, because it is known only through empirical investigation. Following such considerations of Kripke and others (seeHilary Putnam), philosophers tend to distinguish the notion of aprioricity more clearly from that of necessity and analyticity.
Kripke's definitions of these terms diverge in subtle ways from Kant's. Taking these differences into account, Kripke's controversial analysis of naming as contingent anda priori would, according toStephen Palmquist, best fit into Kant's epistemological framework by calling it "analytic a posteriori."[iii]Aaron Sloman presented a brief defence of Kant's three distinctions (analytic/synthetic, apriori/empirical and necessary/contingent), in that it did not assume "possible world semantics" for the third distinction, merely that some part ofthis world might have been different.[9]
The relationship between aprioricity, necessity and analyticity is not easy to discern. Most philosophers at least seem to agree that while the various distinctions may overlap, the notions are clearly not identical: thea priori/a posteriori distinction isepistemological; the analytic/synthetic distinction islinguistic; and the necessary/contingent distinction ismetaphysical.[10]
The terma priori isLatin for 'from what comes before' (or, less literally, 'from first principles, before experience'). In contrast, the terma posteriori isLatin for 'from what comes later' (or 'after experience').
They appear in Latin translations ofEuclid'sElements, a work widely considered during theearly European modern period as the model for precise thinking.
An early philosophical use of what might be considered a notion ofa priori knowledge (though not called by that name) isPlato'stheory of recollection, related in the dialogueMeno, according to which something likea priori knowledge is knowledge inherent,intrinsic in the human mind.[citation needed]
Albert of Saxony, a 14th-century logician, wrote on botha priori anda posteriori.[11]
The early modernThomistic philosopherJohn Sergeant differentiates the terms by the direction of inference regarding proper causes and effects. To demonstrate somethinga priori is to "Demonstrate Proper Effects from Proper Efficient Causes" and likewise to demonstratea posteriori is to demonstrate "Proper Efficient Causes from Proper Effects", according to his 1696 workThe Method to Science Book III, Lesson IV, Section 7.
G. W. Leibniz introduced a distinction betweena priori anda posteriori criteria for the possibility of a notion in his (1684) short treatise "Meditations on Knowledge, Truth, and Ideas".[12]A priori anda posteriori arguments for the existence of God appear in hisMonadology (1714).[12]
George Berkeley outlined the distinction in his 1710 workA Treatise Concerning the Principles of Human Knowledge (para. XXI).
The 18th-century German philosopherImmanuel Kant (1781) advocated a blend ofrationalist andempiricist theories. Kant says, "Although all our cognition begins with experience, it does not follow that it arises from [is caused by] experience."[13] According to Kant,a priori cognition istranscendental, or based on theform of all possible experience, whilea posteriori cognition is empirical, based on thecontent of experience:[13]
It is quite possible that our empirical knowledge is a compound of that which we receive through impressions, and that which the faculty of cognition supplies from itself sensuous impressions [sense data] giving merely theoccasion [opportunity for a cause to produce its effect].
Contrary to contemporary usages of the term, Kant believes thata priori knowledge is not entirely independent of the content of experience. Unlike therationalists, Kant thinks thata priori cognition, in its pure form, that is without the admixture of any empirical content, is limited to the deduction of theconditions of possible experience. Thesea priori, or transcendental, conditions are seated in one's cognitive faculties, and are not provided by experience in general or any experience in particular (although an argument exists thata priori intuitions can be "triggered" by experience).
Kant nominated and explored the possibility of atranscendental logic with which to consider the deduction of thea priori in its pure form.Space,time andcausality are considered purea priori intuitions. Kant reasoned that the purea priori intuitions are established via histranscendental aesthetic and transcendental logic. He claimed that the human subject would not have the kind of experience that it has were thesea priori forms not in some way constitutive of him as a human subject. For instance, a person would not experience the world as an orderly, rule-governed place unless time, space and causality were determinant functions in the form of perceptual faculties, i. e., there can be no experience in general without space, time or causality as particular determinants thereon. The claim is more formally known as Kant'stranscendental deduction and it is the central argument of his major work, theCritique of Pure Reason. The transcendental deduction argues that time, space and causality are ideal as much as real. In consideration of a possible logic of thea priori, this most famous of Kant's deductions has made the successful attempt in the case for the fact ofsubjectivity, what constitutes subjectivity and what relation it holds with objectivity and the empirical.
After Kant's death, a number of philosophers saw themselves as correcting and expanding his philosophy, leading to the various forms ofGerman Idealism. One of these philosophers wasJohann Fichte. His student (and critic),Arthur Schopenhauer, accused him of rejecting the distinction betweena priori anda posteriori knowledge:
... Fichte who, because thething-in-itself had just been discredited, at once prepared a system without any thing-in-itself. Consequently, he rejected the assumption of anything that was not through and through merely ourrepresentation, and therefore let the knowingsubject be all in all or at any rate produce everything from its own resources. For this purpose, he at once did away with the essential and most meritorious part of theKantian doctrine, the distinction betweena priori anda posteriori and thus that between thephenomenon and the thing-in-itself. For he declared everything to bea priori, naturally without any evidence for such a monstrous assertion; instead of these, he gave sophisms and even crazy sham demonstrations whose absurdity was concealed under the mask of profundity and of the incomprehensibility ostensibly arising therefrom. Moreover, he appealed boldly and openly tointellectualintuition, that is, really toinspiration.
— Schopenhauer,Parerga and Paralipomena, Vol. I, §13
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