This thread was inspired by discussion with
@Ponder This in the thread about Aquinas’s teleological argument which is also related to intellect.
Greeks had a concept called
nous. It's the highest faculty of the human soul, responsible for our ability to grasp the essences of things and to reason on the basis of them. This enables us to attain truth and understand the world around us.
Take for example what we know about triangles:
The Pythagorean theorem, etc., were true long before we discovered them and will remain true long after we’re all dead, just as the sun and planets were here before we were and would remain even if we blasted ourselves out of existence in a nuclear conflagration. Now if the essence of triangularity is something neither material nor mental – that is to say, something that exists neither in the material world nor merely in the human mind – then it has a unique kind of existence all its own, that of an abstract object existing in what Platonists sometimes call a “third realm.” And what is true of the essence of triangles is no less true, in Plato’s view, of the essences of pretty much everything: of squares, circles, and other geometrical figures, but also (and more interestingly) of human beings, tables and chairs, dogs and cats, trees and rocks, justice, beauty, goodness, piety, and so on and on. When we grasp the essence of any of these things, we grasp something that is universal, immaterial, extramental, and known via the intellect rather than senses, and is thus a denizen of this “third realm.” What we grasp, in short, is a Form. (E. Feser, The Last Superstition)
Plato thinks that the intellect, since it can know the forms, must also be something immaterial and also immortal.