St. Thomas Aquinas: On the existence of angels and on guardian angels

What are angels and how do we know of their existence?

Thomas gives an argument that the perfection of the universe requires the existence of intellectual creatures. Since God intends the good for His creation, he intends that it be like Himself. And since an effect is most like its cause when it shares with it the feature whereby it was caused, God's creation must contain something with intellect and will since that is how God creates, i.e. by first knowing it and loving it into being.

Hence the perfection of the universe requires that there should be intellectual creatures. Now to understand cannot be the action of a body, nor of any corporeal power.... Hence the perfection of the universe requires the existence of an incorporeal creature. (ST Ia 50, 1)

However, since humans are intellectual creatures, as he indicates at the end of this very argument, the need for some intellectual creatures is not sufficient to give us knowledge of the existence of purely intellectual creatures which the angels are.

Since Sacred Scripture does speak definitively about the existence of angels, it belongs to Sacred Doctrine, i.e. theology, to treat of angels in a truly scientific manner. The divine science has the intellectual tools (faith in Scripture) to establish both the fact of angels and their nature (ST Ia, 1, 3). Having accepted on faith that angels exist, or taking their existence to be purely hypothetical, one can still draw certain philosophical conclusions about their nature. Thomas' words in the Summa are an excellent guide for how one can think clearly about the angelic hosts.

James Collins, The Thomistic Philosophy of

the Angels, a Dissertation,

(Washington, D.C.: Catholic University Press, 1947)

Summa Theologiae > First Part >Question 113. The guardianship of the good angels

Article 1: It is written (Psalm 90:11): "He hath given His angels charge over thee, to keep thee in all thy ways."

It is manifest that as regards things to be done human knowledge and affection can vary and fail from good in many ways; and so, it was necessary that angels should be deputed for the guardianship of men, in order to regulate them and move them to good.

By free-will man can avoid evil to a certain degree, but not in any sufficient degree; forasmuch as he is weak in affection towards good on account of the manifold passions of the soul. Likewise, universal natural knowledge of the law, which by nature belongs to man, to a certain degree directs man to good, but not in a sufficient degree; because in the application of the universal principles of law to particular actions man happens to be deficient in many ways. Hence it is written (Wisdom 9:14): "The thoughts of mortal men are fearful, and our counsels uncertain." Thus, man needs to be guarded by the angels.