The meaning of this impossibility has been explained in Chap. XXX, and appears again in the last argument of this chapter. For the doctrine that "subsistent intelligences [angels] are pure subsistent forms" see B. I, Chap. XLIV, n. 7, with note. The Platonic 'idea,' existing apart from things, was personified by the Neoplatonists, and became a daimon (spirit). But in becoming a spirit it still remained a self-subsistent 'idea,' or 'form,' to the Neoplatonist. The schoolmen held the doctrine of angels as part of the Christian revelation. But being much influenced by Neoplatonism through Arabian and other channels, they came to say of angels some things that the Neoplatonists had said of daimones. The angel then ipsa forma subsistens, it was substantia separata, it was a pure substantial form subsisting by itself. It stood in sharp contrast with Aristotelian 'forms' that were in matter, the most noteworthy of which was the human soul, the 'form of the body.'
Of God and His Creatures: 2.55