A priori

From RationalWiki

A priori is a Latin term used in formal logic (and philosophy) to mean a fact that is assumed to be true prior to any reasoning or research.

At the most solipsistic level, the only a priori statement that a tough skeptic can be assured of is "I think, therefore I am" ("cogito ergo sum"),^[1] although it can be argued that even Descartes' thought experiment had the a priori assumption that a thought requires a thinker. In the real world, most well-documented, objective, observable events will be considered a priori.

Science is a grand effort to build strong a posteriori theories on this framework of observed events.

Thus, the a priori observation that objects fall towards the earth has been hypothesized about and experimented on, producing the a posteriori theories about gravitation.