Copleston: The existence of angels

"THAT ANGELS EXIST, St. Thomas considered to be rationally provable, quite apart from revelation, for their existence is demanded by the hierarchic character of the scale of being. We can discern the ascending orders or ranks of forms from the forms of inorganic substances, through vegetative forms, the irrational sensitive forms of animals, the rational soul of man, to the infinite and pure Act, God; but there is a gap in the hierarchy. The rational soul of man is created, finite and embodied, while God is uncreated, infinite and pure spirit: it is only reasonable, then, to suppose that between the human soul and God there are finite and created spiritual forms which are without body. At the summit of the scale is the absolute simplicity of God: at the summit of the corporeal world is the human being, partly spiritual and partly corporeal: there must, therefore, exist between God and man beings which are wholly spiritual and yet which do not possess the absolute simplicity of the Godhead.”

~Frederick Copleston: A History of Philosophy, Vol. II, Chap. XXXIII.

Also see St. Thomas's Disputed Questions on Spiritual Creatures (especially A. 5.).

Recommended reading: The Angels and Us, by Mortimer J. Adler.

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"Angel" by Gian Lorenzo Bernini. Marble, 1667-69; Ponte Sant'Angelo, Rome.