Full Idea
I am much mistaken if there is anyone who dares to say that he can conceive of a purely possible substance, …for although one talks so much of them, one never conceives them except according to the notion of those which God has created.
Gist of Idea
No one can conceive of a possible substance, apart from those which God has created
Source
Antoine Arnauld (Letters to Leibniz [1686], 1686.05.13), quoted by David Wiggins - Sameness and Substance 4.2
Book Reference
Wiggins,David: 'Sameness and Substance' [Blackwell 1980], p.104
A Reaction
This idea cashes out in the 'necessitism' of Tim Williamson, and views on the Barcan formulae in modal logic.