Plotinus  ’ discussion of perception and of the human soul in general has a notoriously Aristotelian air, although influence from certain Platonic passages is also in evidence. For instance, the Aristotelian notions of perception as the reception of the form of the perceived object, and of perceptions as acts pervade Plotinus’ account of perception. On fundamental issues in psychology, however, Plotinus’ position is unquestionably Platonic: he consistently maintains the immortality of the soul and an essential distinction between the soul and the body along with a doctrine about the soul’s kinship with a realm of transcendent forms. Hence, Aristotelian hylomorphism is unacceptable to him, and he has to modify the Aristotelian notions which he employs, to fit his dualistic views. Similarly, though to a lesser extent, Plotinus is influenced by Stoicism. In his account of the soul and perception we find him adopting some Stoic notions, for example the concepts of sympatheia and synaisthesis. But the Stoics are more relevant as Plotinus’ main antagonists. One can see, for instance, that Stoic physicalist views on the soul and perceptual impressions prompt Plotinus to articulate some novel views on these subjects, and that in general Stoic materialism helps to polish and sharpen Plotinus’ dualistic views.

Although the details remain to be discussed, the foregoing should make clear that Plotinus’ views on perception are a synthesis of many elements in Greek philosophy. In fact, like many other aspects of his philosophy, they present a paradigm case of how philosophical ideas can be moulded and swayed in the course of history so that in a new context they come to do a different job from the one they were originally intended to do. It is not my primary concern to trace Plotinus’ sources, nor do I intend to relate the history of the notion of perception over a period. However, it is often crucial for an understanding of Plotinus’ words to pay attention to his sources and to the historical context of his work; and this I attempt to take into account.

Much of what Plotinus has to say about perception is directly connected with the general dualistic view of the relation between the soul and the body that he wants to maintain. Dualism is of course a part of Plotinus’ Platonic heritage, but, as we shall see as we go on, Plotinus’ formulation of it is sharper and gives rise to questions which Plato never thought of. Plotinus takes it for granted that perception is a phenomenon in which both body and soul are involved. This is of course not at all original: all Greek philosophers would have agreed. What distinguishes Plotinus from most of his predecessors is his view that the soul is neither a body of any kind nor some sort of a principle of the body, but something which is not to be defined in terms of a body at all.

This generates some special problems. For Plotinus it becomes important to distinguish between the soul’s part in perception and the part played by the body. It is Plotinus’ view that the soul cannot be affected by the body. However, he believes along with everyone else that perception involves an affection of the percipient from the outside. It therefore becomes important to make a sharp distinction between the affection, which is attributed to the bodily organ of sense, and what the soul does. But although Plotinus makes it clear that such a distinction must be made, it is not as clear exactly what the affection is and what it is that the soul does. To find this out is crucial, however, if we are to reach an understanding of Plotinus’ views. So much is clear, however, that in Plotinus’ view the soul’s act in perception, though connected with the percipient’s body and with an external object, is itself of a non-bodily nature. Thus we encounter in Plotinus’ theory -perhaps for the first time in western philosophy - a view according to which perception is a matter of crossing an ontological rift between the mental and the physical.

I mentioned above that I take as my starting point statements about perception that either recur in the Enneads or are somehow emphasized. Some of these have already been mentioned on the preceding pages. But I shall now list the most important of these statements, and comment briefly on each of them.

(1) Vision and hearing do not depend on the affection of a medium between percipient and object. What other philosophers explained in terms of a medium, Plotinus explains in terms of the phenomenon of sympatheia (cf. iv.5. 1-4; 8). The notion of sympatheia is Stoic in origin, although apparently the Stoics did not use it in their account of perception. While the doctrine of sympatheia does not play a central role in Plotinus’ theory of perception (in the sense that it is not much in evidence outside Ennéade IV.5, which specifically deals with it), this doctrine is nevertheless the background to other passages. And since Plotinus’ discussion of sympatheia and the question of medium in Ennéade IV.5 brings to light several important aspects of his views on perception, it seems to be a suitable starting point for an account of these views.

(2) If there is to be sense-perception, a bodily sense-organ must be affected by the object perceived. All Greek philosophers who had anything to say about perception gave their assent to this statement or one close to it. Thus Plotinus is in no way saying anything original here. It is, however, doubtful that all Greek philosophers had precisely the same thing in mind when they said that perception must involve the affection of a sense-organ. We will have to try to determine exactly what Plotinus means when he says this and what role this affection plays in his theory as a whole.

(3) In perception the percipient receives the form of the object perceived. This doctrine is Aristotelian, but there has been much disagreement among commentators as to what Aristotle   meant by it. In many respects it raises the same questions of interpretation in Plotinus as it does in Aristotle. One line of interpretation of this doctrine in Aristotle is to identify the reception of the form with the affection of a sense-organ: the organ becomes assimilated to the object by becoming physically qualified in the same way as it, i.e. by taking on its form. Another is to interpret the statement that the sense receives the form of the object (without its matter) as a way of saying that the object comes to exist in the percipient as an intentional object of the same sort. Plotinus’ version of this doctrine seems to lend itself to various interpretations in a similar way. Perception as a reception of forms is a recurring theme that is connected with almost everything else Plotinus has to say about perception.

(4) Certain facts about the unity of perception reveal characteristics of the soul which serve to mark an ontological distinction between the soul and the body. Plotinus’ position on the unity of perception is, I think, best understood through a consideration of the views of his predecessors. I begin with a survey of earlier views, in particular those of Alexander of Aphrodisias  . I argue that Plotinus’ position is a natural, but still original and interesting, development of Alexander’s views. Plotinus says that perception terminates in the faculty of representation (phantasia). This raises certain questions about the distinction and division of labour between perception and the faculty of representation that has a bearing on the interpretation of Plotinus’ views on the unity of perception.

(5) Perceptions are a kind of judgement (krisis). This doctrine indicates that Plotinus is one of those thinkers who take sense to be a cognitive power in its own right and not merely an instrument of cognitive powers. But if perceptions are supposed to be judgements, we must inquire what distinguishes them from other judgements.

(6) Perceptions are acts (energeiai) as opposed to affections. The view that perceptions are acts is derived from Aristotle, as I noted above. But it is not clear that Plotinus and Aristotle have exactly the same thing in mind when they describe perceptions as energeiai. I compare Plotinus’ and Aristotle’s views on this matter and argue that they are probably not identical. Plotinus’ view that perceptions are acts is closely connected with the view that perceptions are judgements, and both of these are connected with the issue of forms in perceptions.

(7) Plotinus insists that what we perceive is the things themselves. Nevertheless, as we shall see, the question arises whether he held some sort of representational view of perception familiar from modern philosophy, according to which there is a "veil of ideas" between the percipient and the external world. Some such view has in fact been attributed to Plotinus, and, as we shall see later on, there are indeed passages that lend themselves to such an interpretation. However, I do not think that this interpretation is correct.

The relationship between these themes - external objects of perception, affections, judgements, acts and forms in perception - is discussed where I set forth what I take to be Plotinus’ theory of perception as a whole. Finally, I discuss Plotinus’ version of dualism of soul and body as it emerges from his account of perception. The origin and history of the mind-body distinction is a matter that has received considerable attention in recent years. In particular it has been debated whether dualistic views of the Cartesian type were ever conceived by the ancients. Although Plotinus’ views are not exactly Cartesian, they exemplify a sharper and subtler dualistic position than any previous Greek thinker had arrived at.

In general I use the words "perception" and "perceive" to translate the Greek words aisthesis and aisthanesthai. It is a well known fact that aisthesis covers both perception and sensation. I think, however, that so far as the Enneads are concerned "perception" is on the whole a more appropriate translation. But that does not necessarily mean that it is always so, and there are indeed places where it is debatable which translation should be used.

Nevertheless, I stick to "perception" as a rendering of aisthesis throughout, except when aisthesis - then used with the definite article - clearly refers to the faculty of perception. In those cases I either use "faculty of perception" or "sense". This choice is determined solely by a wish to have a uniform translation of this key word: and, as I have said, if one is to choose between the two, "perception" is preferable. On the other hand, following custom, I talk about the senses, sense-organs, sensibles and so on, in rendering words of the same root as aisthesis.