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Latin term meaning “from the cause to the effect.” A priori is a term of logic used to denote that when one generally accepted truth is shown to be a cause, another particular effect must necessarily follow. This phrase refers to a type of reasoning that examines given general principles to discover what particular facts or real-life observations can be derived from them. Another name for this method is deductive reasoning.
However, modern usage has deviated significantly from the Latin. An a priori conclusion or judgment is one that is necessarily true, that is neither proved by nor capable of being disproved by experience, and that is known to be true by a process of reasoning independent of all factual evidence. The term is commonly used to indicate a judgment that is widely believed to be certain or that is introduced presumptively, without analysis or investigation. Thus to accuse someone of having assumed a fact or conclusion a priori is often to disparage him or her for having failed to support a judgment through evidence or analysis.