A priori (from that which is before), in Logic, a method of reasoning from a general principle to a particular cause or effect. Mathematical proofs are a priori, and, the data being hypothetical, the reasoning is quite trustworthy. In other cases, however, a priori reasoning is very apt to be fallacious. "A priori Knowledge" is a term applied by Kant and others to knowledge alleged to be involved in the structure of the mind itself, and not derived from, but only suggested by, experience : e.g. the knowledge that 2x2 = 4, or that every change has a cause.