A priori:
Locke and Hume argued that all our ideas or concepts are derived from experience.
Against this, others, such as Plato and Leibniz, have contended that there are
some notions of great importance - for instance, substance, equality, cause,
or likeness and difference - that could not have been so derived; these notions
are on this account rated a priori concepts. It has also been argued, above
all by Kant, that some such a priori concepts are presupposed by the very possibility
of experience...
A
Dictionary Of Philosophy. Second edition. (Pan Books 1984)
Definitions