in Western philosophy since the time of Immanuel
Kant, knowledge that is independent of all particular experiences, as
opposed to
a posteriori knowledge, which derives from experience alone. The Latin phrases
a priori (“from what is before”) and
a posteriori (“from
what is after”) were used in philosophy originally to distinguish
between arguments from causes and arguments from effects.
The first recorded occurrence of the phrases is in the writings of the 14th-century logician
Albert of Saxony.
Here, an argument a priori is said to be “from causes to the effect”
and an argument a posteriori to be “from effects to causes.” Similar
definitions were given by many later philosophers down to and including
G.W. Leibniz, and the expressions still occur sometimes with these
meanings in nonphilosophical contexts. It should be remembered that
medieval logicians used the word “cause” in a syllogistic sense
corresponding to Aristotle's
aitia and did not necessarily mean by
prius something earlier in time. This point is brought out by the use of the phrase
demonstratio propter quid (“demonstration on account of what”) as an equivalent for
demonstratio a priori and of
demonstratio quia (“demonstration that, or because”) as an equivalent for
demonstratio a posteriori. Hence
the reference is obviously to Aristotle's distinction between knowledge
of the ground or explanation of something and knowledge of the mere
fact.
Latent in this distinction for
Kant
is the antithesis between necessary, deductive truth and probable,
inductive truth. The former applies to a priori judgments, which are
arrived at independently of experience and hold universally; the latter
applies to a posteriori judgments, which are contingent on experience
and therefore must acknowledge possible exceptions. In his
Critique of Pure Reason
Kant used these distinctions, in part, to explain the special case of
mathematical knowledge, which he regarded as the fundamental example of
a priori knowledge.
Although the use of a priori to distinguish
knowledge such as that which we have in mathematics is comparatively
recent, the interest of philosophers in that kind of knowledge is
almost as old as philosophy itself. No one finds it puzzling that one
can acquire information by looking, feeling, or listening, but
philosophers who have taken seriously the possibility of learning by
mere thinking have often considered that this requires some special
explanation.
Plato maintained in his
Meno and in his
Phaedo that
the learning of geometrical truths was only the recollection of
knowledge possessed in a previous existence when we could contemplate
the eternal ideas, or forms, directly.
Augustine
and his medieval followers, sympathizing with Plato's intentions but
unable to accept the details of his theory, declared that the ideas
were in the mind of God, who from time to time gave intellectual
illumination to men.
René Descartes,
going further in the same direction, held that all the ideas required
for a priori knowledge were innate in each human mind. For Kant the
puzzle was to explain the possibility of a priori judgments that were
also synthetic (
i.e., not merely explicative of concepts), and the solution that he proposed was the doctrine that space, time, and the categories (
e.g., causality), about which we were able to make such judgments, were forms imposed by the mind on the stuff of experience.
In
each of these theories the possibility of a priori knowledge is
explained by a suggestion that we have a privileged opportunity for
studying the subject matter of such knowledge. The same conception
recurs also in the very un-Platonic theory of a priori knowledge first
enunciated by
Thomas Hobbes in his
De Corpore and
adopted in the 20th century by the logical empiricists. According to
this theory, statements of necessity can be made a priori because they
are merely by-products of our own rules for the use of language.
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