Full Idea
If essence and existence were two things, then no contradiction would be involved if God preserved the essence of a thing in the world without its existence, or vice versa, its existence without its essence; both of which are impossible.
Gist of Idea
If essence and existence were two things, one could exist without the other, which is impossible
Source
William of Ockham (Summa totius logicae [1323], III,II,c,xxvii)
Book Reference
Ockham,William of: 'Ockham's Philosophical Writings', ed/tr. Boehner,P [Hackett 1990], p.93
A Reaction
Not that William is using the concept of a supreme mind as a tool in argument. His denial of essence as something separable is presumably his denial of the Aristotelian view of universals, as well as of the Platonic view.