The first is that our mind can see material things not through themselves, but only through representative beings which are distinct from our perceptions and must precede them, and to which he gave the name 'ideas,' though it was a misuse of the word.
The second is that our mind can find those ideas, or beings representative of material things, only in God.
The third is that what makes it possible for the mind to find them in God is that God contains in himself an infinite intelligible extension.
Antoine Arnauld, On True and False Ideas, Kremer, tr. Edwin Mellen (1990) p. 106. The summary is fairly decent, as one would expect of Arnauld, although one can quibble a bit with both the selection and the unqualified statements.