Thomas Aquinas was so called, because he discussed the knotty points in connection with the being and nature of angels. An example is, “Utrum Angelus moveatur de loco ad locum transeundo per medium? ” The Doctor says that it depends upon circumstances.
It is said, by way of a quiz, that one of his questions was: “How many angels can dance on the point of a pin?”
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894