  But this argument by itself is not sufficient to determine that God necessarily exists. Leibniz must also show that existence is itself a perfection, so that a being having all perfections, an ens perfectissimum, may be said to exist. More exactly, Leibniz needs to show that necessary existence belongs to the essence of God. And this he does in another short piece from this period, writing “Again, a necessary being is the same as a being from whose essence existence follows. For a necessary being is one which necessarily exists, such that for it not to exist would imply a contradiction, and so would conflict with the concept or essence of this being” (A VI iii 583/SR 107). In other words, if it is the case that a necessary being is the same thing as a being whose existence follows from its essence, then existence must in fact be one of its essential properties. Leibniz continues in this short reflection, “And so existence belongs to its concept or essence. From this we have a splendid theorem, which is the pinnacle of modal theory and by which one moves in a wonderful way from potentiality to act: If a necessary being is possible, it follows that it exists actually, or, that such a being is actually found in the universe” (A VI iii 583/SR 107). The “pinnacle of Modal Theory” that Leibniz mentions here is none other than one of the notorious axioms of the modal logic S5: ◊□p → □p. In short, Leibniz's argument is the following: