Full Idea
A substance is something that could exist as a subject but never as a mere predicate.
Gist of Idea
A substance could exist as a subject, but not as a mere predicate
Source
Immanuel Kant (Critique of Pure Reason [1781], B149)
Book Reference
Kant,Immanuel: 'Critique of Pure Reason', ed/tr. Guyer,P /Wood,A W [CUO 1998], p.256
A Reaction
Interesting to see Kant asserting the idea of substance a century after many philosophers thought they had dispensed with this Aristotelian notion (e.g. Ideas 3628 and Idea 2714). It has crept back into modern metaphysics too (e.g. in Wiggins).
Related Ideas
Idea 3628 Substance cannot be conceived or explained to others [Gassendi on Descartes]