Boethius explains this in his little tractate, The Trinity is One God not Three Gods:
But the Divine Substance is Form without matter, and is therefore One, and is its own essence. But other things are not simply their own essences, for each thing has its being from the things of which it is composed, that is, from its parts. It is This and That, i.e. it is the totality of its parts in conjunction; it is not This or That taken apart. Earthly man, for instance, since he consists of soul and body, is soul and body, not soul or body, separately; therefore he is not his own essence. That on the other hand which does not consist of This and That, but only of This, is really its own essence, and is altogether beautiful and stable because it is not grounded in any alien element.