Extension is a basic and immediately known reality. Bodies are extended, space has extension, time has length. We are familiar with the mathematical account of it. What about the metaphysical account?
Metaphysics analyses extension in light of the identity of the whole and the perfect (or act). Thus, extension has infinite imperfection, so far as by its very nature it admits of unlimited division. In natural extension, this division is not always infinite (though it remains infinite in motion and becoming) But there is nevertheless a necessity of division in all extension, and so a necessity of imperfection.
This imperfection is first overcome by that which, though extended, has its whole being in every division or part, that is, by life. In what lives, to exist is to live, and every part of that which lives is alive. Thus we divide mere extension from life, for the metaphysician knows life by its overcoming or transcendence of extension.
But if life transcends mere extension, there is nothing surprising in the fullest life transcending extension altogether. This fullest life is merely perfect and whole by itself, not existing in extended parts. Such subsistence is life, or that which transcends life. This is the existence of intelligence.
But extension serves as an analogue for other sorts of division that we find among intelligences. Just as we must transcend any division into extended parts, so too we must transcend the division of one intelligence from another. We attain to an intelligence that must be utterly sui generis, that is, of itself and in no way one of an enumerated plurality. This is the intelligence of the angel.