Relating to or derived by reasoning from self-evident propositions.
Her colleagues rejected Professor Winslow's a priori argument because it rested on assumptions they felt were not necessarily true.
A priori is Latin for "from the former"; it is traditionally contrasted with a posteriori. It is usually applied to lines of reasoning or arguments that proceed from the general to the particular or from causes to effects. Whereas a posteriori knowledge is knowledge based solely on experience or personal observation, a priori knowledge is knowledge derived through the power of reasoning. An a priori argument is based on reasoning from what is self-evident; it does not rely on observed facts for its proof.