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EMPIRICAL KNOWLEDGE
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 posteriori knowledge is knowledge that is dependent on experience, while a priori knowledge is independent of experience.
Introduction
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 "aprioricty" are sometimes used as nouns (roughly) to 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, the relevent notions involved are 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.
History of usage
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.
Rationalists and empiricists
Aprioricity first became widely debated among rationalist and empiricist philosophers during the Early Modern Period. Rationalists argued that most, if not all, knowledge is acquired a priori, while empiricists argued that all knowledge is ultimately derived from some kind of experience. The French rationalist René Descartes (1641) asserted that the human mind is equipped with a "natural light", a sort of perception-like form of pure reason that acquaints one with certain truths without appeal to experience. 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 between ideas" and "matters of fact", respectively).
Kant
In seeking to show that the debate between rationalists and empiricists was misguided, the German philosopher Immanuel Kant (1781) aligned a priori knowledge with necessary truths. He argued that experience cannot provide us with knowledge of necessities. Thus, according to Kant, all knowledge of necessities must be a priori; this necessity is supposed to be either logical (as holds of what Kant calls analytic truths) or cognitive (part of the way in which minds synthesize the materials presented to them by experience).
Necessary a posteriori
Since Kant, philosophers tended to conflate a priori knowledge with analytic and necessary truths. However, the American philosopher Saul Kripke (1972) argued against Kant's claim that only a priori truths are necessary. 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 necessary (since water and H2O are the same thing, they are identical in every possible world) and a posteriori (since it is known only through empirical investigation). Since Kripke, Hilary Putnam and other philosophers have more clearly separated the notion of a priori from that of necessity.
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 it as an illegitimate distinction that ought to be discarded. Many naturalistic philosophers follow Quine in his skepticism about a priori knowledge; however, Quine's critique is highly controversial among contemporary philosophers.
References and further reading
- 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. (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.
- Kant, Immanuel. (1781). Critique of Pure Reason, trans. N.K. Smith (London: Macmillan, 1929). 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.)
- Plato. (380 B.C.E.). Meno, in Plato: Complete Works, Cooper, J. M. (ed.), Indianapolis: Hackett, 1997. Online text
- Quine, W. V. (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.)
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