A PRIORI.—Reasoning from what is prior either as a condition of thought, or as a
condition of existence—prior, logically or chronologically. (1) Reasoning from
cause to effect (Aristotle); (2) from first truths, self-evident, and essential
to intelligence; (3) from the forms of cognition which are independent of
experience (Kant). According to Kant, à priori applies to forms of knowledge
which are prior in logical order to experience.
"The term à priori, by the influence of Kant and his school, is now very
generally employed to characterise those elements of knowledge which are not
obtained à posteriori—are not evolved out of factitious generalisations;
but which as native to, are potentially in, the mind antecedent
to the act of experience, on occasion of which (as constituting
its subjective condition) they are first actually elicited into
Previously to Kant the terms
priori and à posteriori were, in a sense which descended from Aristotle,
properly and usually employed—the former to denote a reasoning from cause to
effect—the latter a reasoning from effect to cause. The term à priori came,
however, in modern times, to be extended to any abstract reasoning from a given
notion to the conditions which such a notion involved; hence, for example, the
title à priori bestowed on the ontological and cosmological arguments for the
existence of the Deity. The latter of these, in fact, starts from
experience—from the observed contingency of the world, in order to construct the
supposed notion on which it founds. Clarke's cosmological demonstration called
priori, is therefore, so far, properly an argument à posteriori" (Sir W.
Hamilton, Reid's Works, p. 762).
"There are two general ways of reasoning, termed arguments
à priori and à
posteriori, or according to what is usually styled the synthetic and
method; the one lays down some previous self-evident principles; and, in the
next place, descends to the several consequences that may be deduced from them;
the other begins with a view of the phenomena themselves, traces them to their
original, and by developing the properties of these phenomena, arrives at the
knowledge of the cause" (King, Essay on Evil, preface, p. 9).
"Of demonstrations there are two sorts; demonstrations à priori, when we
argue from the cause to the effect; and à posteriori, when we argue from the
effect to the cause. Thus when we argue from the ideas we have of immensity,
eternity, necessary existence, and the like, that such perfections can reside
but in one being, and thence conclude that there can be but one supreme God, who
is the cause and author of all things, and that therefore it is contradictory to
this to suppose that there can be two necessary independent principles, the one
the cause of all the good, and the other the cause of all the evil that is in
the world; this is an argument à priori" (Dr John Clark, Enquiry into Evil, pp.
"By knowledge à priori," says Kant (Critique of Pure Reason, introd., sec. 1),
"we shall in the sequel understand, not such as is independent of this or that
kind of experience, but such as is absolutely so of all experience. Opposed to
this is empirical knowledge, or that which is possible only à posteriori, that
is, through experience. Knowledge à priori is either pure or impure. Pure
knowledge à priori is that with which no empirical element is mixed up. For
example, the proposition 'Every change has a cause,' is a proposition à priori,
but impure, because change is a conception which can only be derived from
"If there are any truths which the mind possesses, whether consciously or
unconsciously, before and independent of experience, they may be called à priori
truths, as belonging to it prior to all that it acquires from the world around.
On the other hand, truths which are acquired by observation and experience are
called à posteriori truths, because they come to the mind after it has become
acquainted with external facts. How far à priori truths or ideas are possible is
the great campus philosophorum, the great controverted question of mental
philosophy" (Thomson, Outline of Laws of Thought, 2nd ed., p. 68; 3rd ed., p.
"It must be noticed that the term à priori has undergone important changes of
meaning. In Aristotle's philosophy, the general truth is 'naturally prior'
(πρότερον τῇ φύσει) to the particular, and the cause to the effect; but since
we know the particular before the universal, and the effect before we seek the
cause, the particular and the effect are each prior in respect to us (πρότερον
πρὸς ἡμᾱς)" (Anal. Post, I. 2 ; Top., VI. 4;
Metaph. V. (Δ), XI. 1018, ed.
Berol; Thomson's Outlines of the Laws of Thought, 3rd ed., p. 68).— V. DEMONSTRATION.