A priori and a posteriori (philosophy)

The terms "a priori" and "a posteriori" are used in philosophy to distinguish between two different types of propositional knowledge. Thus, attempts to clearly define or explain a priori and a posteriori knowledge are part of a central thread in epistemology, the study of knowledge. Since the definitions and usage of the terms are disputed and have evolved in the history of philosophy, it is difficult to provide proper definitions of them. Rough and oversimplified explanations are as follows: a priori knowledge is independent of experience, while a posteriori knowledge is dependent on experience.

Usage of the terms
The terms "a priori" and "a posteriori" are primarily used among philosophers to refer to two different types of knowledge. Thus, they are primarily used as adjectives to modify the noun "knowledge", or taken to be compound nouns that refer to types of knowledge (for example, "a priori knowledge"). However, "a priori" is sometimes used as an adjective to modify other nouns, such as "truth". Additionally, philosophers often modify this usage when beneficial to do so. For example, "apriority" and "aprioricity" are sometimes used as nouns to (roughly) refer to the phenomena that is a priori knowledge. Note that it is common to italicize "a priori" and "a posteriori" since they derive from the corresponding Latin phrases. However, since they have become technical terms in philosophy, some do not feel the need to italicize the terms or separate the "a" from the rest of the term.

The intuitive distinction
Although definitions and usage of the terms have varied in the history of philosophy, they have been consistently intended to demarcate two separate epistemological notions. The intuitive distinction between a priori and a posteriori knowledge can be seen in examples of what is supposed to fall under each concept. To borrow from Jerry Fodor (2004), take, for example, the proposition expressed by the sentence, "George V reigned from 1910-1936". This is something that one must come to know a posteriori (assuming that it is knowledge), because it expresses an empirical fact that one cannot come to know of by reason alone. By contrast, consider the proposition expressed by the sentence, "If George V reigned at all, then he reigned for a while". This is something that one knows a priori, because it expresses a fact that is non-empirical and that one can come to know by reason alone.

Early uses
The phrases "a priori" and "a posteriori" are Latin in origin, which literally mean "from what comes before" and "from what comes later", respectively (or, less literally, "before experience" and "after experience", respectively). An early philosophical use of what might be considered a notion of a priori knowledge is Plato’s theory of recollection, related in the the dialogue Meno (380 B.C.E.), according to which something like a priori knowledge is knowledge inherent in the human mind.

Rationalism and empiricism
The nature of a priori and a posteriori knowledge first became widely debated among rationalist and empiricist philosophers during the early modern period. The two camps primarily used the terms "a priori" and "a posteriori" to differentiate between how knowledge is acquired or derived. The Rationalists, on one hand, argued that most, if not all, knowledge is acquired a priori, not via experience. The empiricists, on the other hand, argued that all knowledge is ultimately derived a posteriori, derived directly from experience. The French rationalist René Descartes, for example, asserted that the human mind is equipped with a "natural light", a form of pure reason that acquaints one with certain truths without appeal to experience: "...for what the natural light shows to be true can be in no degree doubtful, as, for example, that I am because I doubt, and other truths of the like kind.... Similarly, the German rationalist Gottfried Leibniz categorized all knowledge into two categories: "truths of reason" and "truths of fact". Leibniz writes:

There are also two kinds of truths, those of reasoning and those of fact. Truths of reasoning are necessary and their opposite is impossible: truths of fact are contingent and their opposite is possible. When a truth is necessary, its reason can be found by analysis, resolving it into more simple ideas and truths, until we come to those which are primary.

The British empiricist John Locke (1689), on the other hand, argued that the human mind is a tabula rasa, a "blank slate", onto which experience impressed the materials for all knowledge. The Scottish empiricist David Hume (1777), although he never explicitly used the terms, seemed to consider all knowledge to be either a priori or a posteriori, which he called "relations of ideas" and "matters of fact", respectively. Relations of ideas, according to Hume, are "...discoverable by the mere operation of thought, without dependence on what is anywhere existent in the universe...." Thus, the rationalists and empiricists largely based their opposition to each other on their disagreement about the nature of a priori and a posteriori knowledge.

Immanuel Kant
The German philosopher Immanuel Kant (1781) advocated a blend of rationalist and empiricist theories. Kant claims: "That all our knowledge begins with experience there can be no doubt... But... it by no means follows that all arises out of experience." According to Kant, a priori knowledge is transcendental, or based on the form of all possible experience, while a posteriori knowledge is empirical, based on the content of experience. Kant states, "...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 giving merely the occasion)". Thus, unlike the empiricists, Kant thinks that a priori knowledge is independent of the content of experience; however, unlike the rationalists, Kant thinks that a priori knowledge is limited to knowledge of the possibility of experience, which is seated in one's cognitive faculties, not any particular experience. Kant concluded that one's ability to acquire a priori knowledge, then, must involve an extra-empirical cognitive faculty, which he called pure intuition.

Relation to the analytic-synthetic


Several philosophers reacting to Kant sought to explain a priori knowledge without appealing to, as Paul Boghossian explains, "a special faculty...that has never been described in satisfactory terms". One theory, which was especially popular among the logical positivists of the early twentieth century, is what Boghossian calls the "analytic explanation of the a priori". The distinction between analytic and synthetic propositions was first introduced by Kant. While Kant's original distinction was primarily drawn in terms of conceptual containment, the contemporary version of the distinction primarily involves, as Quine put it, the notions of "true by virtue of meanings and independently of fact." Analytic propositions are thought to be true in virtue of their meaning alone, while synthetic propositions are thought to be true in virtue of their meaning and certain facts about the world. According to the analytic explanation of the a priori, all a priori knowledge is analytic; so a priori knowledge need not require a special faculty of pure intuition, since it can be accounted for simply by one's ability to understand the meaning of the proposition in question. In short, proponents of this explanation claimed to have reduced a dubious metaphysical faculty of pure reason to a legitimate linguistic notion of analyticity.

However, the analytic explanation of a priori knowledge has undergone several criticisms. Most notably, the American philosopher W. V. O. Quine (1951) argued that the analytic-synthetic distinction is illegitimate (see Quine's rejection of the analytic-synthetic distinction). Quine states: "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." While the soundness of Quine's critique is highly disputed, it had a powerful affect on the project of explaining the a priori in terms of the analytic.

Relation to the necessary/contingent
The metaphysical distinction between necessary and contingent truths has also been related to a priori and a posteriori knowledge. A proposition that is necessarily true is one whose negation is self-contradictory (thus, it is said to be true in every possible world). Consider the proposition that all bachelors are unmarried. Theoretically, its negation, the proposition that some bachelors are married, is incoherent, because the concept of being unmarried (or the meaning of the word "unmarried") is 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, because it is impossible for them to be true. Thus, the negation of a self-contradictory proposition is supposed to be necessarily true. By contrast, a proposition that is contingently true is one whose negation is not self-contradictory (thus, it is said that it is not true in every possible world). As Jason Baehr states, it seems plausible that that all necessary propositions are known a 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."

Following Kant, some philosophers have considered the relationship between aprioricity, analyticity, and necessity to be extremely close. According to Jerry Fodor, "Positivism, in particular, took it for granted that a priori truths must be necessary...." However, since Kant, the distinction between analytic and synthetic propositions had slightly changed. Analytic propositions were largely taken to be "true by virtue of meanings and independently of fact", while synthetic propositions were not&mdash;one must conduct some sort of empirical investigation, looking to the world, to determine the truth-value of synthetic propositions.

However, aprioricity, analyticity, and necessity have since been more clearly separated from each other. The American philosopher Saul Kripke (1972), for example, provided strong arguments against this position. Kripke argued that there are necessary a posteriori truths, such as the proposition that water is H2O (if it is true). According to Kripke, this statement is necessarily true (since water and H2O are the same thing, they are identical in every possible world, and truths of identity are logically necessary) and a posteriori (since it is known only through empirical investigation). Following such considerations of Kripke and others (such as Hilary Putnam), philosophers tend to more clearly distinguish the notion of aprioricity from that of necessity and analyticity.

Thus, the relationship between aprioricity, necessity, and analyticity is not easy to discern. However, most philosophers at least seem to agree that while the various distinctions may overlap, the notions are clearly not identical: the a priori/a posteriori distinction is epistemological, the analytic/synthetic distinction is linguistic, and the necessary/contingent distinction is metaphysical.

Controversies
Currently, there are still debates concerning the nature of aprioricity and related notions among philosophers. Some, such as the American philosopher W. V. O. Quine (1951), regard talk of aprioricity as illegitimate. Many naturalistic philosophers follow Quine in his skepticism about a priori knowledge; nonetheless, Quine's critique is highly controversial.

References and further reading

 * Baehr, Jason. (2006). "A Priori and A Posteriori", Internet Encyclopedia of Philosophy. Online text
 * Boghossian, Paul. (1997). "Analyticity Reconsidered", Nous, vol. 30, no. 3, pp. 360-391. Online text
 * Boghossian, P. & Peacocke, C., eds. (2000). New Essays on the A Priori, Oxford: Oxford University Press.
 * Descartes, René. (1641). Meditations on First Philosophy. In Cottingham, et al. (eds.), The Philosophical Writings of Descartes, Cambridge University Press, 1984. Online text
 * Fodor, Jerry. (1998). Concepts: Where Cognitive Science Went Wrong, New York: Oxford University Press.
 * Fodor, Jerry. (2004). "Water's water everywhere", London Review of Books, Vol. 26, No. 20, dated 21 October 2004. Online text
 * Hume, David. (1777). An Enquiry concerning Human Understanding, Nidditch, P. N. (ed.), 3rd. ed., Oxford: Clarendon Press, 1975. Online text
 * Kant, Immanuel. (1781). Critique of Pure Reason, trans. N.K. Smith (London: Macmillan, 1929). Online text
 * Kant, Immanuel. (1783). Prolegomena to any Future Metaphysics, Paul Carus (trans.). Online text
 * Kripke, Saul. (1972). "Naming and Necessity", in Semantics of Natural Language, edited by D. Davidson and G. Harman, Boston: Reidel. (Reprinted in 1980 as Naming and Necessity, Cambridge, MA: Harvard University Press.)
 * Leibniz, Gottfried. (1714). Monadology, in Philosophical Essays, edited and translated by Roger Ariew and Daniel Garber, Indianapolis: Hackett, 1989. Online text
 * Locke, John. (1689). An Essay Concerning Human Understanding, Prometheus Books. Online text
 * Plato. (380 B.C.E.). Meno, in Plato: Complete Works, Cooper, J. M. (ed.), Indianapolis: Hackett, 1997. Online text
 * Quine, W. V. O. (1951). "Two Dogmas of Empiricism", The Philosophical Review, Vol. 60, pp. 20-43. (Reprinted in Quine's From a Logical Point of View, Harvard University Press, 1953.) Online text