Book III discusses the mind or rational soul, which belongs to humans alone. He argues that thinking is different from both sense-perception and imagination because the senses can never lie and imagination is a power to make something sensed appear again, while thinking can sometimes be false. And since the mind is able to think when it wishes, it must be divided into two faculties: one which contains all the mind's ideas which are able to be considered, and another which brings them into act, i.e. to be actually thinking about them. These are called the possible and agent intellect. The possible intellect is an "unscribed tablet" and the store-house of all concepts, i.e. universal ideas like "triangle", "tree", "man", "red", etc. When the mind wishes to think, the agent intellect recalls these ideas from the possible intellect and combines them to form thoughts. The agent intellect is also the faculty which abstracts the "whatness" or intelligibility of all sensed objects and stores them in the possible intellect. For example, when a student learns a proof for the Pythagorean theorem, his agent intellect abstracts the intelligibility of all the images his eye senses (and that are a result of the translation by imagination of sense perceptions into immaterial phantasmata), i.e. the triangles and squares in the diagrams, and stores the concepts that make up the proof in his possible intellect. When he wishes to recall the proof, say, for demonstration in class the next day, his agent intellect recalls the concepts and their relations from the possible intellect and formulates the statements that make up the arguments in the proof.