There is a vast lacuna in nature, if nothing in the scale of being intermediates between man and God. It may be said that disembodied spirits so intermediate, especially if we allow, what St Thomas does not, that, once parted from the body, the soul expands into a perfect spiritual substance, with no remaining natural exigency of reunion with the body. Yet even so, if spirits exist which have been in bodies, why not other spirits which never have been in bodies? Throughout this argument we cannot travel beyond congruity. For the fact of the existence of angels we require either experience, which we have not, or divine revelation, which we have.
Of God and His Creatures: 2.91