For metaphysicians who have imbibed the sober and inebriating teachings of Thomas Aquinas, existence is an act, the act which makes all things actually to be. As the act of existence makes things to be, essence makes them to be what they are. Essence and the act of existence, in other words, are really distinct yet together they compose each of the things that are.

Such an understanding involves a number of paradoxes, and Frederick D. Wilhelmsen's articulation of them reveals his philosophical genius. These paradoxes include the fact that the act of existence does not exist, that it...