Well, any Thomist will want to emphasize, first, that being in one way, rather than another, is different from being at all, rather than not being. That, put rather crudely, is the difference between essence and existence. If the difference between them is really rooted in reality, and not just in the mind, then it is at least plausible to posit really distinct principles to account for them: not only an essence-principle, but also an existence-principle.
