"The world of pure spirits"

“GOD’s universal providence works through secondary causes. . . . The world of pure spirits stretches between Divine Nature and the world of human beings; because Divine Wisdom has ordained that the higher should look after the lower, angels execute the divine plan for human salvation: they our guardians, who free us when hindered and help to bring us home.”

~St. Thomas Aquinas: Commentary on the Sentences, 2, 11, 1, 1.

Linaioli Tabernacle (detail), by Fra Angelico.
Tempera on wood, c. 1433;
Museo di San Marco, Florence.

"The higher spirits are also called angels"

"THUS, the higher spirits are also called angels, because they direct the lower spirits, as it were, by bringing messages to them; in fact, angels are spoken of as messengers. And they are also called ministers, because they carry out by their operation the order of divine providence even in the area of bodily things. Indeed, a minister is “like a living instrument,” according to the Philosopher [Politics I, 4: 1253b 29]. So this is what is said in the Psalm (103:4): “You make your angels spirits, and your ministers a burning fire.”"

 ~St. Thomas Aquinas: Contra Gentes, 3, 79.