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A priori (literally "from before") is a Latin term used in formal logic (and philosophy) to mean a fact that is assumed to be true prior to any empirical research. A priori conclusions are usually reached through reasoning rather than observation and are the centerpiece of (philosophical) rationalism. It is opposed to so-called a posteriori knowledge, which is gained through observation and forms the basis for empiricism.
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"), although it can be argued that even Descartes' thought experiment had the a priori assumption that a thought requires a thinker.
In science, the deduction of hypotheses can be described as an effort resting on a priori knowledge, while theory development and testing necessitate a posteriori knowledge.