Full Idea
Extension and impenetrability together constitute the concept of matter.
Gist of Idea
Extension and impenetrability together make the concept of matter
Source
Immanuel Kant (Critique of Pure Reason [1781], B646/A618)
Book Reference
Kant,Immanuel: 'Critique of Pure Reason', ed/tr. Guyer,P /Wood,A W [CUO 1998], p.576
A Reaction
Descartes had settled for extension alone. Kant's simple claim is probably now just a historical footnote, as we would now turn to physicists to define matter. Extension might survive, but impenetrability is not a key notion in quantum mechanics.