Summa Theologica

This treatise is important in Aquinas’ scheme because, while scripture describes the action and function of the angels, it does not describe what angels actually are. In his discussion, Aquinas assumes that angels are created beings who are intelligent and spiritual (non-material, incorporeal). Aquinas starts by discussing why it is necessary to assert the existence of angels in the universe. 

The angels must be purely spiritual creatures, because the perfection of the universe requires that there be such creatures. God is pure spirit, and he is the cause of all effects, which resemble him as their cause. Also, the goal of any creature is to be assimilated into (or return to) God, and this is most possible for a purely spiritual creature. Further, the fact that it is possible for us to apprehend things purely by the mind, not the senses, shows that there must be things grasped through the intellect alone. 

Thus, the angels rank between God (pure spirit) and corporeal creatures (man and animals). Angels are incorruptible because they subsist in their own being, which nevertheless depends on God as its cause.