Latin.  From the cause to the effect.  A Latin term of logic that is used to denote that when one generally accepted truth is shown to be a cause, another particular effect must necessarily follow.

WordNet 3.6

  • adj. a priori involving deductive reasoning from a general principle to a necessary effect; not supported by fact “an a priori judgment”
  • adj. a priori based on hypothesis or theory rather than experiment
  • adv. a priori derived by logic, without observed facts

Webster’s Revised Unabridged Dictionary

A priori (Philos) Applied to knowledge and conceptions assumed, or presupposed, as prior to experience, in order to make experience rational or possible.“A priori , that is, form these necessities of the mind or forms of thinking, which, though first revealed to us by experience, must yet have preëxisted in order to make experience possible.
A priori (Logic) Characterizing that kind of reasoning which deduces consequences from definitions formed, or principles assumed, or which infers effects from causes previously known; deductive or deductively. The reverse of a posteriori.

Chambers’s Twentieth Century Dictionary

A priori ā pri-ō′rī, a term applied to reasoning from what is prior, logically or chronologically, e.g. reasoning from cause to effect; from a general principle to its consequences; even from observed fact to another fact or principle not observed, or to arguing from pre-existing knowledge, or even cherished prejudices; (Kant) from the forms of cognition independent of experience

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