Intelligence is the faculty of knowing, and its characteristic activity is thought. Ideas are representational, for each idea represents some object. This naturally provides a way to classify the different forms of intellectual activity – that is, there are as many intellectual faculties as there are types of objects to be known.

Humans can have knowledge of three types of objects – those given to us in experience, those given to us outside experience, and finally those of the inner world. Many people question whether there really are things known by us outside of experience, but (without answering the question here) we'll take the commonsense position that recognizes three different kinds of knowledge. We can always reduce these to two if necessary.

This division yields three faculties of perception – the senses, reason, and consciousness.

There are three other intellectual faculties that deal with objects having only a virtual presence. These are the association of ideas, memory, and imagination, which we'll call the faculties of conception.

Finally, beyond these simple faculties, there are also a number of complex operations formed by the combination of different faculties. These are abstraction, attention, judgment, and reasoning.

Such are the major divisions of the theory of knowledge.