Nous (UK: /naʊs/, US: /nuːs/), from Ancient Greek: νοῦς, is a concept from classical philosophy, sometimes equated to intellect or intelligence, for the faculty of the human mind necessary for understanding what is true or real.

Alternative English terms used in philosophy include "understanding" and "mind"; or sometimes "thought" or "reason" (in the sense of that which reasons, not the activity of reasoning). It is also often described as something equivalent to perception except that it works within the mind ("the mind's eye"). It has been suggested that the basic meaning is something like "awareness". In colloquial British English, nous also denotes "good sense", which is close to one everyday meaning it had in Ancient Greece. The nous performed a role comparable to the modern concept of intuition.

In Aristotle's philosophy, which was influential on later conceptions of the category, nous was carefully distinguished from sense perception, imagination, and reason, although these terms are closely inter-related. The term was apparently already singled out by earlier philosophers such as Parmenides, whose works are largely lost. In post-Aristotelian discussions, the exact boundaries between perception, understanding of perception, and reasoning have sometimes diverged from Aristotelian definitions.

In the Aristotelian scheme, nous is the basic understanding or awareness that allows human beings to think rationally. For Aristotle, this was distinct from the processing of sensory perception, including the use of imagination and memory, which other animals can do. For him then, discussion of nous is connected to discussion of how the human mind sets definitions in a consistent and communicable way, and whether people must be born with some innate potential to understand the same universal categories in the same logical ways. Derived from this it was also sometimes argued, in classical and medieval philosophy, that the individual nous must require help of a spiritual and divine type. By this type of account, it also came to be argued that the human understanding (nous) somehow stems from this cosmic nous, which is however not just a recipient of order, but a creator of it. Such explanations were influential in the development of medieval accounts of God, the immortality of the soul, and even the motions of the stars, in Europe, North Africa and the Middle East, amongst both eclectic philosophers and authors representing all the major faiths of their times.

In early Greek uses, Homer used nous to signify mental activities of both mortals and immortals, for example what they really have on their mind as opposed to what they say aloud. It was one of several words related to thought, thinking, and perceiving with the mind. In pre-Socratic philosophy, it became increasingly distinguished as a source of knowledge and reasoning opposed to mere sense perception or thinking influenced by the body such as emotion. For example, Heraclitus complained that "much learning does not teach nous".

Among some Greek authors, a faculty of intelligence known as a "higher mind" came to be considered as a property of the cosmos as a whole. The work of Parmenides set the scene for Greek philosophy to come, and the concept of nous was central to his radical proposals. He claimed that reality as perceived by the senses alone is not a world of truth at all, because sense perception is so unreliable, and what is perceived is so uncertain and changeable. Instead he argued for a dualism wherein nous and related words (the verb for thinking which describes its mental perceiving activity, noein, and the unchanging and eternal objects of this perception, noēta) describe another form of perception which is not physical, but intellectual only, distinct from sense perception and the objects of sense perception.

Anaxagoras, born about 500 BC, is the first person who is definitely known to have explained the concept of a nous (mind), which arranged all other things in the cosmos in their proper order, started them in a rotating motion, and continuing to control them to some extent, having an especially strong connection with living things. (However Aristotle reports an earlier philosopher, Hermotimus of Clazomenae, who had taken a similar position.) Amongst the pre-Socratic philosophers before Anaxagoras, other philosophers had proposed a similar ordering human-like principle causing life and the rotation of the heavens. For example, Empedocles, like Hesiod much earlier, described cosmic order and living things as caused by a cosmic version of love, and Pythagoras and Heraclitus, attributed the cosmos with "reason" (logos).

According to Anaxagoras the cosmos is made of infinitely divisible matter, every bit of which can inherently become anything, except Mind (nous), which is also matter, but which can only be found separated from this general mixture, or else mixed into living things, or in other words in the Greek terminology of the time, things with a soul (psychē).