"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.

Is it fitting for each man to have an angel guardian?

"MAN while in this state of life, is, as it were, on a road by which he should journey towards heaven. On this road man is threatened by many dangers both from within and from without, according to Psalm 141:4: "In this way wherein I walked, they have hidden a snare for me." And therefore as guardians are appointed for men who have to pass by an unsafe road, so an angel guardian is assigned to each man as long as he is a wayfarer. When, however, he arrives at the end of life he no longer has a guardian angel; but in the kingdom he will have an angel to reign with him, in hell a demon to punish him."

~ St. Thomas Aquinas: Summa Theologica, I, q. 113, a. 4

Read more about guardian angels here

The Guardian Angel, by Pietro da Cortana. Oil on canvas, 1656;
Galleria Nazionale d'Arte Antica, Rome.