Full Idea
Notoriously, Locke's filler for Descartes's geometrical matter, solidity, will not do, for that quality collapses on examination into a composite of the dispositional-cum-relational propery of impenetrability, and the secondary quality of hardness.
Gist of Idea
Locke's solidity is not matter, because that is impenetrability and hardness combined
Source
Howard Robinson (Perception [1994], IX.3)
Book Reference
Robinson,Howard: 'Perception' [Routledge 2001], p.220
A Reaction
I would have thought the problem was that 'matter is solidity' turns out on analysis to be a tautology. We have a handful of nearly synonymous words for matter and our experiences of it, but they boil down to some 'given' thing for which we lack words.