"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.