- a priori
- adj. known ahead of time
- adj. based on hypothesis rather than experiment
- a posteriori
- adj. involving deduction of theories from facts.
Appropriated directly from Latin in the early 18th century, these epistemological phrases have become favorites of mine, to the point that my neologistic streak kicks in and I start using them as nouns because they flow so much better. The correct usage would be to modify nouns like “truth”—i.e. “an a priori truth.” But that’s beyond the point.
Literally, the two words mean “from what comes before” and “from what comes after,” respectively (think “prior” and “posterior”); that is, inductive and deductive reasoning. Which is to say, the first is a conclusion from previous knowledge (or more pejoratively, a preconceived notion), whereas the latter is a conclusion from either deductive reasoning or empirical observation.
Immanuel Kant1 was famous for arguing a mixture of the two, inevitably the former as a transcendental sort of cognition and the latter as a kind of experienced content; this was in direct opposition to the more skeptical conclusions of his contemporary David Hume.
Interesting sidenote: though the usage is not conventional, some have taken to deriving nouns from these phrases; hence the words “apriority” and “aposteriority,” slight reworkings of existing words.
- c.f. The Critique of Pure Reason[↩]