Commonly translated as 'mind' or 'intellect', the Greek word nous is a key term in the philosophies of Plato, Aristotle and Plotinus. What gives nous its special significance there is not primarily its dictionary meaning - other nouns in Greek can also signify the mind - but the value attributed to its activity and to the metaphysical status of things that are 'noetic' (intelligible and incorporeal) as distinct from being perceptible and corporeal. In Plato's later dialogues, and more systematically in Aristotle and Plotinus, nous is not only the highest activity of the human soul but also the divine and transcendent principle of cosmic order.
In its pre-philosophical usage nous is only one among a number of terms for mind. It is chiefly distinguished from these other words by its tendency to signify 'intelligent' activity - realizing, understanding, planning, visualizing - rather than mental processes more generally, including the emotions.
The earliest Greek philosophers traded on this usage of the term. Heraclitus complained that 'much learning does not teach nous'. In Parmenides, the cognate verb noein and other related words (noma, 'thought' and notos, 'thinkable') are crucial to his argument. Drawing an absolute distinction between 'opinion' (doxa) based upon sense perception and 'truth' (altheia), Parmenides argues that 'noetic' activity, properly speaking, is indissolubly linked to true discourse, valid reasoning and the cognition of reality. What is not true cannot be spoken or 'thought', a rule that applies not only to 'nothing' but also to the illusory data of sense perception. The reality that Parmenides' nous deduces and apprehends is unqualified 'being', homogeneous and invariant in time and place.
Anaxagoras (§4), with a cosmology strongly influenced by Parmenides, adopted nous as the controlling principle of the universe. Making nous quite separate from everything else, he characterized it as 'the finest and purest of all things, which has all knowledge about everything and the greatest power'. Nous causes the primordial mixture of other things to rotate and separate into distinct beings.
In the dialogues of Plato the treatment of nous was powerfully influenced by these antecedent conceptions. In his Phaedo, Socrates favours the idea that nous has organized the universe in the best possible way (an idea, he suggests, that Anaxagoras failed to carry through). This conception of nous was fully developed in other Platonic dialogues including the Timaeus, where it is figuratively expressed in the teleological thinking of the world's divine manufacturer (demiurge). In the Republic, the three great images of sun, divided line and cave are ways of distinguishing levels of reality and modes of cognition. Common to all three images is a distinction between the visible world of 'unknowable' phenomena and the 'noetic' world of stable and intelligible Forms. Nosis - the highest activity of the soul's rational component - has cognition of the Forms as its objective, which it pursues by seeking understanding that is unhypothetical and absolutely secure. In ethical and psychological contexts Plato also uses nous as a term for the soul's 'rational component', with meanings that may be as broad as 'mind' in everyday English.
Although Plato's special uses of nous left their mark on Aristotle, the latter arrived at systematic ideas concerning nous as the distinctive faculty of the human soul (see Aristotle §19). In Aristotle's general model of the soul, psychic functions are realizations of bodily potentialities. Nous, by contrast, 'has no actual existence before it thinks', and it has no corresponding organ as 'perception' has in being the function of eye, ear, and so on. Like Plato, Aristotle links nous to the thinking of incorporeal 'forms' - the definable essences of things; but in contrast to Plato's independently existing 'Forms', those of Aristotle only become actual 'thought objects' in being thought since nous is identical in its actuality to what it thinks. For Aristotle, in contrast again with Plato, nous can only perform its activity with the help of data provided by 'imagination' (phantasia), which is the soul's capacity to represent sensory information, and it functions as the agent not only of theoretical activity but also of purposive action in everyday life.
In a notoriously obscure chapter (III 5) of his work On the Soul, Aristotle distinguishes nous as 'a capacity to become everything' from nous as 'a capacity to make everything', in the way that light makes potential colours actual. This 'active' nous, called 'immortal', has often been identified with the Aristotelian Unmoved Mover, whose life is 'a thinking of thinking' (see Aristotle §16). But Aristotle probably regarded human thought as being godlike rather than as being a product of the Unmoved Mover, who exists as an eternally transcendent thinker.
For Plotinus (§4), nous comprises 'primary reality', the domain of intelligence and intelligible beings. He construes this domain as an 'emanation' from the ineffable One, the ultimate principle of everything. Taken universally, nous corresponds more or less to a syncretism of Plato's Forms with Aristotle's Unmoved Mover. Everlastingly contemplating the One, nous is construed as an equivalence between thought thinking itself and intelligible beings as the only true thinkables. The activity of nous 'overflows' into 'soul', the principle of embodied life. As a lower level of reality, soul can only think things by treating them successively and separately. Human beings live primarily at the level of 'soul', but they also, by virtue of their immortal and 'undescended' self, have access to identification with nous and thereby to a mode of being in which thinker and thought are completely unified. In this transcendent condition, the mind is reality itself.
Stoics and Epicureans tend to use other words for the mind, probably because as rigorous physicalists they found nous too strongly tinged by Platonic metaphysics.
See also: Alexander of Aphrodisias §2; Marcus Aurelius §2; Neoplatonism §3; Numenius §2; Platonism, Early and Middle §5; Proclus §5; Psych; Ptolemy; Theophrastus §3A.A. LONG