That is a very good question. In Samkhya-Yoga philosophy what is known as the Internal Instrument consists of 3 parts: 1. Manas, which controls the senses and organs of action and the processes of deliberation, and is concerned with the external world. 2. Ahamkara - literally the I-maker, the principle of individuality, the sense of self, the agent or doer, the owner. This is the realm of feeling and passion. 3. Buddhi. Often confused with intellect, Buddhi is ascertainment, the ability to discern the true nature of a thing, referred to in the Yoga Sutras as the power of seeing.

This is very old and would probably be dismissed by modern science, but I think it has validity.