ExplanationNous is the special faculty or part of the soul by which one perceives truth and other things of a more divine nature, such as Virtue. The exercise of the Nous – i.e., knowing things by means of it – is called noesis.
Noesis is often compared to vision; it is something like 'seeing' a truth. When we say, "Ah, now I see your point" and the like, that expresses having a noetic understanding or grasp of a truth or principle.
Today there is no English word in common use that carries the same meaning as Nous. As a result, the concept of the Nous has all but vanished from modern science. Historically, the words Intellect and Intelligence meant much the same thing as Nous; however today that connection is obscured by a very broad meaning of 'intelligence', which can be even applied to animals and machines (because they can, for example, plan and learn). However Nous is something like consciousness itself, and, at least with machines, certainly wouldn't be applicable.
Another possible English near-equivalent is Reason. But here we must distinguish between Reason and reasoning. Reasoning is a form of discursive thought, whereas noesis is direct apprehension of truth. Discursive thought corresponds to what Greek philosophers called dianoesis, which is associated with a different mental faculty, the dianoia. Reason itself (i.e., Nous) is part of reasoning – i.e., the means by which we perceive trueness of an argument or inference.
Noesis is sometimes defined as intuition. Again, this is only partly true, because intuition has two common meanings. The first (the ability to understand something immediately, without the need for conscious reasoning) is similar to noesis; the second (knowledge that comes from instinctive feeling rather than conscious reasoning) is not.
We are left with the practical difficulty that, in terms of common usage, there are no convenient English words with the same meanings as Nous and noesis. Therefore it might be advantageous to simply re-introduce the Greek terms into common use. In the case of noesis, this seems to be gradually happening.