These alleged possibilities (if they are anything) would be among all of the other creatures; they would be actuals. It might of course be the case that Descartes just insists that there are possible existents in addition to actual existents. Alternately, it might be the case that like Spinoza (in Ethics Part IV, definitions three and four), Descartes uses the expression “possible existence” to describe actually-existing creatures, and in a way that is consistent with a denial of non-actual rea