A Priori (n.) is a truth, like mathematics, which derives its necessary character not from experience, but from the inherent structure of our minds, from the natural and inevitable manner in which our minds must operate.
A Priori (n.) is a general truth which must be independent of experience; clear and certain in themselves.
A Priori (n.) is exemplified in mathematical knowledge which is necessary and certain; we cannot conceive of future experience violating it.
A Priori (n.) is important because it makes absolute truth, and absolute science, possible.
A Priori (n.) is knowledge that is independent of sense-experience, whose truth is certain to us even before experience.
A Priori (n.) must be true no matter what our later experience may be; true even before experience.