In what sense did Aquinas use 'the agent intellect' and its operation, called 'abstraction'? In his theory of human understanding, it is necessary to posit an agent intellect. For he thought that proper human intellect is the possible intellect, which could not understand things unless it might be moved by an intelligible object. However, such an intelligible objecct does not have real existence because it is already abstracted in the process of cognition. Therefore, in order to make the intelligible objects to be real we have to posit an agent intellect. Aquinas gives explanations of supposing an agent intellect. But we may find some differences between Summa Theologiae (I. Q. 79. a. 4) and De Aniwa (Q. 4), and the operation of abstraction has neither univocal meaning nor unique mode. For instance, one mode of abstraction means "separation" and the other "generalization". I don't intend to give the complete answer to above ambiguities here. I only show that ontological and analogical ways of thoughts are mixed together and used interchangeably in his books.