Of God and His Creatures

The conclusion might be expressed thus: In every created reality, or actuality, the actualisation, or realisation, is something distinct and separable from the thing actualised, or realised. This is not saying that the actualisation might be taken away, and the thing still remain. The distinction between essence and existence is not physical. But created essence has not such a hold on existence as to be incapable of losing it. This loose hold upon existence is taken by the Thomist school to involve a real distinction between essence and existence in creatures.

Of God and His Creatures: 2.52