Human is both an animal and a spirit. As an animal one possesses a cognitive function that makes one able to adapt oneself to new circumstances and to learn from experience. As a spirit one is endowed with a cognitive function akin to the angelic intellect, which makes one capable of reflecting on one’s own intellectual activity and of being conscious of oneself as a subject.
Human intelligence is neither of these two functions but may be considered a combination of both. In human intelligence, the material element is represented by the power of learning from experience, whereas the formal element consists in the purely spiritual function of self-reflection. Because of its formal element, human intelligence is totally different from its animal counterpart. Human is capable of formally knowing universals and relations, while animal discovers them only materially if at all. Thinking is the typical activity of human intelligence.
The senses perceive, memory recalls, imagination pictures objects before the mind; intelligence alone thinks. How does this entire process work and what exactly is the nature of our human intellect? Our intellect is not material in the way a stone is material; it cannot be seen or touched or photographed like a stone. Our intellect is immaterial: this means, it is intrinsically independent of matter. The brain is that portion of matter which has the closest relation to the operations of our intellect.
We claim, therefore, that our intellect is not intrinsically dependent on our brain, that our brain is not a cause of the operations of our intellect, that it does not think. Since we cannot study our intellect directly, we must turn to its internal operations and show that they are immaterial. For, as a being is, so it acts. If the operations of our intellect are immaterial, their cause, our intellect, must likewise be immaterial.