No metaphor is perfect but some are enough to get us started in understanding St. Thomas’s distinction between essence (what a thing is) and existence (the fact that a thing is). The first comes from Barry Miller, who asks us to think of “the homely analogy of a block of butter that has been cut into a number of parts. Each piece of butter has a different surface or bound. A peculiar thing about bounds is that, although they are real enough, they themselves are totally devoid of thickness: they are not to be mistaken for an enveloping film whether of butter or of any other material whatsoever. Despite their ontological poverty, however, they do have a genuine function, for they serve to distinguish every block from every other block. In that sense they can be said to individuate the blocks they bound.”