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The ancient Greek and Roman philosophers developed the ingredients of most of the conceptions about the soul which have later become influential in the history of philosophy. Plato’s contributions to psychology include highly influential arguments for dualism, whereas Aristotle emphasises a functionalist idea of the soul as the form of a living body. However, both philosophers are far from unambiguous in their theories of the soul, and their importance is by no means reducible to these basic ideas.

Plato is famous for his arguments for the soul’s simplicity, non-changeability, immateriality, and divinity in the Phaedo. These characteristics of the human soul run through the whole history of philosophy, and even today it is these properties which often come to mind when the soul is talked about. However, Plato himself seems to have changed his mind about the nature of the soul, or he came to realise that the view presented in the Phaedo was not the whole story. This can be seen, for example, when Plato discusses issues of health and disease. In these contexts he does not always follow strict dualism; rather, a different, much more monistic conception of the soul seems to emerge. In Republic IV, Plato establishes another equally influential conception of the soul, based on its division into three parts or aspects. What was the whole soul in the Phaedo is now regarded as the reasoning part in a tripartite structure which also includes emotions and appetites as faculties of the lower parts of the soul. The Phaedrus and the Timaeus introduce further modifications to Plato’s psychology (1).

As with Plato, with Aristotle there is no real scholarly consensus on the essential nature of his theory of the soul. There is discussion on whether Aristotle’s psychology should be understood in terms of dualism, materialism, functionalism or any other position currently recognised in the philosophy of mind. The problems do not only follow from differences between ancient and modern terminology and categories: Aristotle seems also to operate with different conceptions of the soul in different contexts. His standard view of the body-soul relationship is often called ‘hylomorphism’. The soul is understood as the form of a living material body, its organization for actualizing a set of functional capacities related to nutrition, perception, and thinking. However, in his discussion of the theoretical intellect, there are at least traces of a conception in which the soul and the body are seen as two distinct entities and the soul is not fully reduced to the psychophysical unity of the living body. There are also passages in Aristotle in which the soul is assumed to be located somewhere in the body (2).

Among the Hellenistic philosophers, there existed a remarkable consensus about some features on the nature of the soul, even though their other philosophical views were quite divergent. The Epicureans, the Stoics, and many Hellenistic physicians endorsed dualist theories of the soul in the sense that the soul and the body are distinct from each other as substances. Despite this distinction, many Hellenistic philosophers also held that souls are material or corporeal. They share the belief that something can be said to exist only if it is spatially extended, three-dimensional, and capable of acting or being acted upon. Therefore, the idea of a purely immaterial soul is rejected. Souls have matter which is, however, different from the matter of inanimate, or ‘non-souled’, bodies (3). For Epicurus, the soul is a corporeal and material body but constituted by matter which is different from the rest of the body, i.e., the bones, the muscles and the blood. The soul has to be corporeal since only then can it interact with the rest of the body and be co-affected with it. The Epicureans located the functions of thinking and emotions in the mind, which they located in the chest (or heart), whereas the other functions of the soul extend throughout the body.

The sources of the Stoic position on the human soul is much less clear, but Tertullian (160–220) and Calcidius (fourth century) both testify that Zeno (333–264 BCE) and Chrysippus (279–206 BCE) argued that the soul was pneuma (in Latin spiritus) or breath, and that this is a kind of body. It is of interest that both Epicurus and the Stoics likened the soul with breath. It is this breath that accounts for all the powers of the soul, that is, nutrition, growth, locomotion, sensations, and will.

The later part of the ancient philosophical tradition saw a renewed interest in Plato and Aristotle. The first major commentator of Aristotle, Alexander of Aphrodisias (third century), developed the Aristotelian position in great detail. The most important development was the interpretation he gave to De anima III.5, and the introduction of the so-called agent intellect. The influential discussion of this doctrine is in a small treatise on the intellect which was translated into Latin and known in the Arabic philosophical tradition.

The Platonic doctrines became more and more influential towards the end of the ancient tradition. Plotinus (204/5–270) and the Neo-Platonists foremost incorporated the view which Plato explicated in the Phaedo. Plotinus in turn had a significant influence on the first major Christian philosopher, Augustine (354–430). In De trinitate, Augustine developed arguments for the incorporeality of the soul, which in turn had an enormous influence on philosophy of mind in a Platonic tradition throughout the Middle Ages and into early modern times. The immediate self-knowledge which he stresses became a characteristic of the soul in this context (4).

1 Platos’s Dualism

a. Then what do we say about the soul? Can it be seen or not?

  • It cannot be seen.

  • So it is invisible …

  • Have we not said some time ago that when the soul makes use of the body for an inquiry, be it through hearing or seeing or some other sense – for to inquire through the body is to do it through the senses – it is dragged by the body to the things which are never the same, and it wanders about and is confused and dizzy, as if it were drunk, because it is in contact with such things? …

  • But when the soul inquires by itself, it passes into the realm of what is pure, everlasting, immortal and unchanging, and being akin to these, it always stays with them whenever it is by itself and not hindered; it ceases to wander about and remains in the same state since it is in touch with such things, and this state is called wisdom …

  • [W]hen the soul and the body are joined together, nature directs the one to serve and to be ruled, and the other to rule and be master. Now, which do you think is like the divine and which like the mortal? Do you not think that the nature of the divine is to rule and to lead and that of the mortal to be ruled and serve?

  • I do.

  • Which does the soul resemble?

  • Clearly, Socrates, the soul is like the divine and the body like the mortal.

  • Consider then, Cebes, whether this is a conclusion from all that has been said: the soul is most like the divine, immortal, intelligible, uniform, indissoluble, always the same as itself, whereas the body is most like the human, mortal, multiform, unintelligible, soluble and never the same. (Plato, Phaedo 79b–80b)

b. [Y]ou ought not to attempt to cure the eyes without the head, or the head without the body; so neither the body without the soul. And this, he [the Thracian king Zalmoxis] said, is the reason why most diseases evaded the physicians of Greece, that they disregarded the whole, which ought to be particularly studied, for if this is not well, it is not possible that the parts are well. For all good and evil, whether in the body or the entire human being, originates, as he said, in the soul and flows from there, as if from the head, to the eyes. And therefore you must treat it first and foremost if the head and body are to be well. (Plato, Charmides 156d–157a)

c. In fact I once heard from wise men that we are now dead and the body is our tomb, and the part of the soul in which the desires are is liable to persuasion and vacillates to and fro. So a smart man, who might have been from Sicily or Italy, played with words and called this part a jar because it was so gullible and easily persuaded. (Plato, Gorgias 493a)

d. But a city seemed to be just when each of the three classes of natures within it did its own work, and it was thought to be moderate, courageous, and wise … Then, if a single man has these same forms in his soul, we will expect him to be correctly called by the same names as the city because of these same conditions in them …

  • Well, then, I said, we are surely compelled to agree that we have within us the same forms and characteristics as the city. They could not get there from any other place. It would be ridiculous to think that spiritedness did not come into the cities from such individuals who are held to possess it, such as the Thracians, Scythians, and others who live to the north, and the same holds of the love of learning, which is mostly associated with our part of the world, or of the love of money, which one might say is conspicuously found among the Phoenicians and Egyptians …

  • Do we do these things with the same part of ourselves, or do we do them with three different parts? Do we learn with one part, get angry with another, and with some third part desire the pleasures of food, drink, sex, and the others which are akin to them? …

  • It is obvious that the same thing cannot at the same time do or undergo opposites with respect to the same part and in relation to the same thing. So, if we ever find these in them, we know that they are not the same but many. (Plato, Republic IV, 435b–436c)

e. Enough has been said about the immortality of the soul, but this is what we have to say about its form. To tell what it really is would require an utterly divine and lengthy discourse, but to say what it is like is humanly possible and more modest. Let us now do this. We will liken the soul to the composite power of a pair of winged horses and their charioteer. The gods have both horses and charioteers which are themselves both good and of good descent, whereas those of others are mixed. With the human beings, the driver is in control of a pair of horses. Of the horses, one is beautiful and good and of similar breed, while the other is the opposite by both descent and nature. This necessarily means that, in our case, driving is difficult and troublesome. (Plato, Phaedrus 246a–b)

f. As we said at the beginning, all things were in disorder when God made all things proportionate to themselves and others, as far as it was possible to make them to be in harmony and proportion. At this time, they did not participate to any proportionality, except by chance, nor did they correspond to the names we now use of them, such as fire, water, and other such things. He first put all of these into order and then, out of them, put together this universe, which is a single living thing, including all things both mortal and immortal. The demiurge himself constructed the divine ones among them, but ordered his descendants to be the constructors of the mortal ones. They imitated him, and having received the immortal principle of the soul, around which they fashioned a mortal body. They made the whole body a vehicle and constructed within the body another kind of soul which was mortal and contained within it terrible and necessary passions ... In this way, as was necessary, they framed the mortal soul. (Timaeus 69b–d)

Plato’s dualism is most emphasised in the Phaedo, in which he argues that the soul is a simple unified entity which is unchangeable, immaterial, divine, and immortal (a). There is a contrast between the strict dualism of the Phaedo, the Republic, the Phaedrus, and the Timaeus, on the one hand, and the somewhat more monistic assumptions in the Charmides (b) and the Gorgias (c), on the other (see Robinson 2000). In Greek culture of the fourth century BCE, the idea of immortality of the soul was not commonly accepted, as is emphasised by Socrates’ opponents (see, e.g., Phaedo 70a, 77b), but the arguments designed by Socrates in the dialogue became extremely influential in the later history of philosophy (see Bostock 1986; Lorenz 2008). The activities directly ascribed to the soul in the Phaedo are restricted to the cognitive and intellectual features, whereas the emotions and the appetites are interpreted as functions of the ensouled body. The soul is expected to function in an appropriate way if it is to regulate and control the body with its affections and desires. In the Republic, Plato introduces appetite and spirit as the two lower parts of the soul (d). These parts, however, are presented as mortal, unlike the reasoning part; in the Phaedrus, by contrast (e), even the two lower parts are assumed to be immortal. In the Timaeus (f), which is the latest of the dialogues quoted here, Plato returns to the conception according to which the appetitive and passionate parts of the soul are mortal. See also p. 466.

2 Aristotle’s Theory of the Soul as a Form

a. We call one type of being a substance, either as matter (which in itself is not a ‘this’), or as shape or form (in virtue of which a thing is called a ‘this’), or thirdly as that which is compounded of these. Now matter is potentiality and form is actuality. It is actuality in two ways, as in knowledge and as in contemplating.

Bodies are most commonly regarded as substances, especially natural bodies; for they are the principles of other bodies. Of natural bodies some have life and others do not; by life we mean self-nourishment and growth and decay. So every natural body which has life is a substance, and it is a substance as a composite. Since it is a body of such a kind, for it has life, the soul cannot be a body; for the body does not belong to those which are attributed to a substrate, but rather is a substrate and matter. Hence the soul must be a substance as the form of a natural body which potentially has life. But substance is actuality, and thus soul is the actuality of a body of this kind.

But ‘actuality’ is used in two ways: as that of knowledge, and as that of contemplating. It is obvious that the soul is an actuality in the same way that knowledge is; for both sleeping and waking presuppose the existence of the soul, and waking is analogous to contemplating, and sleeping to knowledge, possessed but not employed. In a subject, knowledge is temporally prior in the order of origin. Hence the soul is the first actuality of a natural body which potentially has life. The body so described has organs. Even the parts of plants are organs, although very simple; for example, the leaf shelters the pod and the pod shelters the fruit, while the roots are analogous to the mouth, both serving for taking in food. If, then, we have to speak of something common to all kinds of soul, it is the first actuality of a natural body which has organs. (Aristotle, De anima II.1, 412a6–b6)

b. Therefore, there is no more need to ask whether the body and the soul are one than whether the wax and the impression in it are one or, in general, whether the matter of each thing and that of which it is the matter are one … The soul is an actuality in the same way that the faculty of seeing and the capacity of a tool are actualities. The body, on the contrary, is potentially a being. Just as the pupil and the capacity of seeing make up an eye, in the same way the soul and the body make up an animal. It is clear that neither the soul nor certain parts of it, if it has parts, are separable from the body, for in some cases the actuality is the actuality of parts themselves. However, nothing prevents that some parts are separable since they are not actualities of any parts of the body. It also remains unclear whether the soul is the actuality of the body in the same way as the sailor is the actuality of the ship. (Aristotle, De anima II.1, 412b6–9, 413a1–9)

c. Concerning the intellect and the faculty of contemplation nothing is so far clear, but it seems to be another kind of soul, and it is only this that is separable, just as the eternal is separable from the perishable. (Aristotle, De anima II.2, 413b24–27)

d. And there is an intellect which is such by becoming all things, while there is another which is what it is by producing all things as a kind of disposition, like light, for light makes potential colours into actual colours. This intellect is separable, impassible, and unmixed, as it is essentially activity. (Aristotle, De anima III.5, 430a14–18)

e. It is clear that one has to regard the affection which is generated through perception in the soul, that is, the part of the body which has it, as a kind of image and the state of having this as memory. (Aristotle, De memoria 1, 450a27–28)

f. The only part which animals must have is something that is analogous to the heart, since the sensitive soul and the source of life in all animals belong to something which rules the body and its parts. (Aristotle, De partibus animalium IV.5, 678b1–4)

Aristotle usually interprets the soul as the form of a living material body, organised to actualize a set of functional capacities related to all aspects of its living, nutrition, perception, and thinking (a). He does not, however, quite consistently follow the hylomorphism in his accounts of the soul’s activities. There are a few occasions in which Aristotle emphasises the separability and immateriality of the intellect (besides b, c, and d, see, e.g., De anima III.5, 430a23–26; De generatione animalium II.3, 736b26–28). The remarks about the immortality and eternity of the separable reason may, according to some commentators, indicate the immortality of individual human souls, but in fact, there is very little in our sources to support this interpretation. Aristotle also seems to assume on some occasions (e, f) that the soul is a distinct entity and has a specific location, i.e., the heart. He distinguishes affections which are common to the soul and the body from those which are peculiar to the soul (De somno 1, 453b12; De anima III.10, 433b19–21; De sensu 1, 436a8), and mentions impulses which arrive at the soul or reach the soul (De anima I.4, 408b16–18; De divinatione per somnum 2, 464a10–11). See Shields 2011.

3 Non-dualist Theories

a. Next, we must see, referring to the perceptions and affections (for these will provide the surest conviction), that the soul is a body composed of fine parts which are diffused all over the aggregate and most closely resemble breath blended with heat, in one way like breath and in another like heat. There is also a part which is much finer than these and because of this is more liable to co-affect with the rest of the aggregate. This is shown by the abilities of the soul: its feelings, its ease of motion, its thought processes, and the things the loss of which lead to death.

Further, we must keep in mind that the soul is most responsible for causing sensation. But it would not be thus if it were not somehow confined within the rest of the aggregate. But the rest of the aggregate, though it provides for the soul this causality, itself has a share in this property because of the soul; still it does not have all the features of the soul. Hence on the departure of the soul it loses sense-perception. For it had not this power all in itself, but something else which came into being with it provided it; and this, through the power brought about in itself by its motion, immediately achieved for itself a property of sentience and then gave it to the other, because of their proximity and mutual harmony, as I said … Furthermore, when the whole aggregate is destroyed, the soul is dispersed and no longer has the same powers, nor its motions; hence, it does not then have sensations, either. (Epicurus, Letter to Herodotus, in Diogenes Laertius, Lives of Philosophers X.63–65)

b. He [Cleanthes] also says that neither incorporeal is co-affected with a body nor a body with anything incorporeal but only a body with another body. The soul is co-affected with the body when it is sick and being cut, and so the body with the soul. Thus when the soul is ashamed, the body becomes red, and when the soul is scared, the body turns pale. So the soul is a body. (Nemesius, De natura hominis 2 (78.7–79.2) = SVF 1.518 = LS 45C)

c. Chrysippus says that death is the separation of the soul from the body. But nothing incorporeal ever separates from the body, for what is incorporeal does not touch the body. The soul, however, does touch the body and is separated from it. Therefore the soul is a body. (Nemesius, De natura hominis 2 (81.6–10) = SVF 2.790 = LS 45D)

d. They [the Peripatetics and the Stoics] first state the assumption that the heart is generated first of all. Second, they also believe that the heart generates the other parts as if the constructor of the heart, whoever it is, had ceased to exist. Finally, it follows, they claim, that even the deliberative part of our souls is situated there. (Galen, De foetuum formatione, Kühn 4, 698 = LS 53D)

e. [Diogenes says the following…] Articulate utterances flow from the same source as plain voice, and, therefore, meaningful articulate utterance also flows from there. This is language. Therefore language flows from the same source as plain voice. Plain voice does not have its origins in the head region, but in a lower area, for it is obvious that it comes from the windpipe. Therefore neither does language have its origins in the head region but in a lower area. But it is also true that language is generated from thought, for some people in fact define language as meaningful utterance that comes from thought. It is also plausible that language flows imprinted or as if stamped by means of conceptions in thought, and it is temporally simultaneous with thinking as well as the activity of speaking. Therefore, neither is thought located in the head but in a lower region, most likely somewhere around the heart. (Galen, On the Doctrines of Hippocrates and Plato 2.5.9–13)

f. Then Zeno, defining the soul as the connatural spirit, teaches as follows: that which causes the death of an animal when it departs is a body. But when the connatural spirit departs, the animal dies. But the connatural spirit is a body. Therefore, the soul is a body. (Tertullian, De anima 5.3 (SVF 1.137))

g. Chrysippus says that it is certain that we breathe and live by one and the same thing. And we breathe by the natural spirit. Therefore we live as well by that very spirit. And we live by the soul. Therefore the soul is found to be natural spirit … The parts of the soul flow from their seat in the heart, as though from the source of a spring, and spread through the whole body, continually filling all the limbs with vital spirit, and ruling and controlling them with countless different powers, such as nutrition, growth, locomotion, sensation, the impulse to action. The soul as a whole extends the senses, which are its functions, from the ruling faculty, like branches from a tree, to report what they sense, while it itself like a king passes judgment on their reports. (Calcidius 220 (SVF 2.879, part; LS 53G))

h. Intellect, according to Aristotle, is threefold. One is material intellect; by ‘material’ I do not mean that it is a substrate like matter … but since what it is for matter to be matter is in its power to become all things, then that is material in which this power and potentiality is, insofar as it is potential … Another is the intellect which is already thinking and has a competence for thinking and is capable of acquiring by its capacity the forms of the objects of thought. It is analogous to those who have the competence for building and are capable by themselves of doing things in accordance with their art … The third intellect, in addition to the two already described, is the productive intellect through which the material intellect receives its competence, and this agent intellect is analogous, as Aristotle says, to light. For as light is the cause which makes potentially visible colours actually visible, so also this third intellect makes the potential and material intellect an actual intellect by instilling a thinking competence in it … The productive intellect is also said to come ‘from outside’, and it is not a part or capacity of our soul, but comes to be in us from outside when we grasp it. (Alexander of Aphrodisias, De intellectu (106.19–108.13))

In Hellenistic philosophy, both the Epicureans and the Stoics held that there is some grounds for distinguishing soul from the body, but only in the sense that the soul is a body which consists of a particular kind of matter (von Staden 2000). The Stoics argued for the corporeality of soul saying that the soul is a body because only bodies have a capacity to affect and be affected by one another (b), and souls and bodies affect one another in occasions of physical pains and emotions (on this argument, see, e.g., Annas 1992a). Epicurus also used the same line of argument (a). The Epicureans and the Stoics were also in agreement in their views that the soul is a particularly fine piece of body, the so-called pneuma (Lat. spiritus), a hot breath which is diffused throughout the living organism (f, g). The Epicureans held that the soul is mortal and dissolves at death (a), whereas the Stoic view was that even though the soul survives death it is mortal in the end (c). As physicians such a Herophilus performed human dissection and possibly also vivisection in Hellenistic Alexandria, new empirical knowledge made it possible to locate the soul in the brain, but the Stoics still subscribed to the heart-centered theory of the soul’s location (d, e). See Tieleman 1996.

The short comment by Aristotle in De anima III.5 alluding to a distinction between the material and the productive part of the intellective soul seems innocent, but has generated intense commentary throughout the history of philosophy beginning with Alexander of Aphrodisias. He draws (h) a threefold distinction, but the first two are usually taken to be the same intellect only taken differently, that is, in one way in potency and in another in act. The active productive intellect is not in the human soul but belongs to the prime mover; cf. Alexander of Aphrodisias, De anima 88.14- 90,19. For discussions of the authenticity of the De intellectu and Alexander of Aphrodisias’ view of the intellect, see Sharples 2008. In late medieval thought Alexander was regarded as a proponent of the view that the human intellectual soul is mortal. See pp. 30–31.

4 Late Ancient Views

a. If this [the soul] were extended, and the perceptions were, as it were, projected onto both extremes of a line, it will be the case that either they will come back together again at a single point, such as the middle, or each of them will have a perception of its own, just as if I perceived something and you something else. And if there is a single thing perceived, such as a face, either of the following will be the case. It will be contracted in a single point, as it appears to happen, for it is gathered together in the pupils of the eyes, for how could we otherwise see large objects through them? Furthermore, in this case what reaches the ruling faculty will be like objects of thoughts and without parts, and the ruling faculty is itself without parts. Or alternatively, if it [the thing perceived] were a magnitude, what perceives would be divisible in the same way, so that each of its parts would apprehend a different part, and nothing in us would have an apprehension of it as a whole. (Plotinus, Enneads IV.7.6.15–26)

b. And, if one ought to have courage to state one’s view more clearly, even if it contradicts the opinion of others, even our soul does not completely come down, but something of it will always remain in the intelligible. If the part which is in the perceptible gains control, or even more if it is controlled or thrown into confusion, we shall not be able to perceive those objects which the upper part of the soul contemplates. The intelligible arrives within our reach, when it comes down to be perceived in its descent. We recognise, for example, an appetite which remains in our appetitive faculty, but only when we apprehend it either by our internal perceptual or intellectual faculty, or by the both of them. (Plotinus, Enneads IV.8.8.1–8)

c. But since we study the nature of the mind, let us remove from our consideration any knowledge which is obtained from without through the senses of the body, and pay more attention to the principle which we have laid down: that all minds know and are certain concerning themselves … Who would doubt that he lives, remembers, understands, wills, thinks, knows, and judges? For even if he doubts, he lives; if he doubts, he remembers why he doubts; if he doubts, he understands that he doubts; if he doubts, he wishes to be certain; if he doubts, he thinks; if he doubts, he knows that he does not know something; if he doubts, he judges that he ought not to consent rashly … And those do not realise that the mind knows itself even when it seeks for itself, as we have shown. But it is not at all correct to say that a thing is known while its substance is unknown. Therefore, when the mind knows itself, it knows its own substance, and when it is certain about itself, it is certain about its own substance. But it is certain about itself, but it is not at all certain whether it is air, or fire, or a body, or something of a body. Therefore, it is none of these things … The mind thinks of fire as it thinks of air or any other bodily thing it thinks of. But it cannot happen that it should think of that which it itself is, in the same way as it thinks of that which it itself is not. For all these, whether fire, or air, or this or that body, or that part or combination or tempering of a body, it thinks of by means of an imaginary fantasy, nor is it said to be all of these, but one or the other of them. But if it were any one of them, it would think of this one in a different manner from the rest. (Augustine, De trinitate X.10.14–16)

Plotinus returns to the strong dualism found in Plato’s Phaedo. He argues against the Stoics that the soul, as distinct from the bodies, is not extended and immaterial. This is taken to be clear from the unity of the subject of perception (a). The subject of perception is not the highest part of the person; it is the intellect through which persons can engage in non-discursive thinking and which does not descend into the body, remaining eternally in higher spheres (b). Later Neoplatonists tended to reject the idea of an undescended part of the soul. See Sorabji 2005, 93–99. Augustine was influenced by Plotinus and argues by way of two related arguments for the incorporeality of the soul. One argument takes its starting point in the soul’s immediate knowledge of itself while the other one argues that if the mind had any particular corporeal nature, it should think of that nature without a representation (c). See Matthews 2003 and Lagerlund 2008.