Being means anything and everything that in any way is, and can at all be said to be removed from the merest nothing. There is being in thought, conceptual, or ideal being; and there is being of thing, -- actually existent being. Being in this latter sense of what actually exists cannot be a genus, because the whole apparatus of genus, species and differentia belongs to the business of definition; and definition does not lay down actual existence (esse), but ideal being (essentia. It is no part of the definition of a triangle to state that any such things as triangles do actually exist. Therefore we read in this chapter (n. 3): "The existence of each thing that exists in a genus is something over and above the quiddity of the genus." In other words, 'existence' lies outside every possible generic notion. Nor again can being in the sense of what is in thought be a genus, because such conceptual being penetrates and pervades the whole ideal order, to which genus, species and differentia belong: it is the fundamental notion of the order, and appears everywhere, and therefore cannot be screened off as a genus. -- See Metaphysics in the Stonyhurst Series of "Manuals of Catholic Philosophy," pp. 35-38.
Of God and His Creatures: 1.25