Full Idea
Strictly speaking no concept given a priori can be defined, e.g. substance, cause, right, equity, etc.
Gist of Idea
No a priori concept can be defined
Source
Immanuel Kant (Critique of Pure Reason [1781], B756/A728)
Book Reference
Kant,Immanuel: 'Critique of Pure Reason', ed/tr. Guyer,P /Wood,A W [CUO 1998], p.638
A Reaction
A passing remark with large and interesting implications. A huge amount of ink has been spilled over whether to take concepts such as identity, truth, goodness and substance as 'basic', or reduce them to something else.