Full Idea
The peripatetic philosophers carry their fictions still further, and both suppose a substance supporting, which they do not understand, and an accident supported, of which they have as imperfect an idea. The whole system is entirely incomprehensible.
Gist of Idea
Aristotelians propose accidents supported by substance, but they don't understand either of them
Source
David Hume (Treatise of Human Nature [1739], I.IV.3)
Book Reference
Hume,David: 'A Treatise of Human Nature', ed/tr. Selby-Bigge/Nidditch [OUP 1978], p.222
A Reaction
It seems to me that if you put it to Aristotle that he didn't understand 'substantial form', he would concede the point, but nevertheless say that it was ideal at which knowledge aimed. Locke is much more astute than Hume on this.