Note 76 — Chapter IV · THE WORLD OF THINGS—(1) SUBSTANCE, QUALITY, AND RELATION

Strictly speaking, the “solidity” or “impenetrability” of the ultimate particles of matter, which is with Locke and Newton one of the most prominent “primary” qualities, is not a “mathematical” property, but it still owes its inclusion in the list to the conviction of these philosophers that it is, like extension and form, fundamentally important for mathematical Physics. The explanation of the “secondary” qualities as subjective appears to go back to Democritus.