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Since commenter zosima used the term a priori I figured it would be worth giving a quick and dirty definition for those who are unfamiliar. It’s a concept that comes up a lot in philosophy, so expect to see more of it.
An a priori judgment is one you can make that is founded on nothing but reason. The best example of this is Descartes’ Meditations, which most of you are probably familiar with—in it, Descartes (pictured) tries to make as many judgments as he can starting from a position of total skepticism about the world around him, his own perception, and even his own existence. In other words, it is a work dedicated to testing the limits of his a priori knowledge.
A posteriori knowledge is the opposite: knowledge that can only be gained through external observation. All knowledge gained through scientific experimentation and empirical observation is a posteriori.
A large portion of epistemology—the philosophical study of the nature of knowledge—is dedicated to figuring out what knowledge we can have a priori and what can only be learned a postiori.