“On the Soul” by Aristotle

Of all the books by Aristotle that I’ve read so far, this is the most fascinating in terms of the depth and scope of the concepts, spanning philosophy, epistemology, physics and neurobiology in their nascent form; Rhetoric, OTOH, is the most entertaining, in terms of psychological insight and perspective.

Substance and Property

It seems not only useful for the discovery of the causes of the derived properties of substances to be acquainted with the essential nature of those substances (as in mathematics it is useful for the understanding of the property of the equality of the interior angles of a triangle to two right angles to know the essential nature of the straight and the curved or of the line and the plane) but also conversely, for the knowledge of the essential nature of a substance is largely promoted by an acquaintance with its properties.

In all demonstration a definition of the essence is required as a starting-point, so that definitions which do not enable us to discover the derived properties, or which fail to facilitate even a conjecture about them, must obviously, one and all, be dialectical and futile.

What is thinking?

If thinking is like perceiving, it must be either a process in which the soul is acted upon by what is capable of being thought, or a process different from but analogous to that. The thinking part of the soul must therefore be, while impassible, capable of receiving the form of an object; that is, must be potentially identical in character with its object without being the object. Mind must be related to what is thinkable, as sense is to what is sensible.

Therefore, since everything is a possible object of thought, mind in order, as Anaxagoras says, to dominate, that is, to know, must be pure from all admixture; for the co-presence of what is alien to its nature is a hindrance and a block: it follows that it too, like the sensitive part, can have no nature of its own, other than that of having a certain capacity. Thus that in the soul which is called mind (by mind I mean that whereby the soul thinks and judges) is, before it thinks, not actually any real thing. For this reason it cannot reasonably be regarded as blended with the body: if so, it would acquire some quality, e.g. warmth or cold, or even have an organ like the sensitive faculty: as it is, it has none. It was a good idea to call the soul ‘the place of forms’, though (1) this description holds only of the intellective soul, and (2) even this is the forms only potentially, not actually.

Observation of the sense-organs and their employment reveals a distinction between the impassibility of the sensitive and that of the intellective faculty. After strong stimulation of a sense we are less able to exercise it than before, as e.g. in the case of a loud sound we cannot hear easily immediately after, or in the case of a bright colour or a powerful odour we cannot see or smell, but in the case of mind thought about an object that is highly intelligible renders it more and not less able afterwards to think objects that are less intelligible: the reason is that while the faculty of sensation is dependent upon the body, mind is separable from it.

Once the mind has become each set of its possible objects, as a man of science has, when this phrase is used of one who is actually a man of science (this happens when he is now able to exercise the power on his own initiative), its condition is still one of potentiality, but in a different sense from the potentiality which preceded the acquisition of knowledge by learning or discovery: the mind too is then able to think itself.

[–Update in 2014–]

Affections of Soul

If there is any way of acting or being acted upon proper to soul, soul will be capable of separate existence; if there is none, its separate existence is impossible.

Affections of soul are enmattered formulable essences. Consequently their definitions ought to correspond, e.g. anger should be defined as a certain mode of movement of such and such a body (or part or faculty of a body) by this or that cause and for this or that end. That is precisely why the study of the soul must fall within the science of Nature, at least so far as in its affections it manifests this double character.

What is Soul?

Those who have special regard to the fact that what has soul in it is moved, identify soul with what is eminently originative of movement. On the other hand, those who look to the fact that what has soul in it knows or perceives what is, identify soul with the principle or principles of Nature.

Plato in the Timaeus fashions soul out of his elements; for like, he holds, is known by like, and things are formed out of the principles or elements, so that soul must be so too. Similarly also in his lectures ‘On Philosophy’ it was set forth that the Animal-itself is compounded of the Idea itself of the One together with the primary length, breadth, and depth, everything else, the objects of its perception, being similarly constituted. Again he puts his view in yet other terms: Mind is the monad, science or knowledge the dyad (because it goes undeviatingly from one point to another), opinion the number of the plane, sensation the number of the solid; the numbers are identified with the Forms themselves or principles, and are formed out of the elements; now things are apprehended either by mind or science or opinion or sensation, and these same numbers are the Forms of things.

Democritus says soul and mind are one and the same thing, and this must be one of the primary and indivisible bodies, and its power of originating movement must be due to its fineness of grain and the shape of its atoms; of all the shapes the spherical is the most mobile, most adapted to permeate everywhere, and to set all the others moving by being themselves in movement, therefore, this is the shape of the particles of fire and mind, and respiration is the characteristic mark of life.

Anaxagoras specially posits mind as the principle of all things; mind alone of all that is simple, unmixed, and pure. He assigns both characteristics, knowing and origination of movement, to the same principle, when he says that it was mind that set the whole in movement.

Diogenes held the soul to be air because he believed air to be finest in grain and a first principle; therein lay the grounds of the soul’s powers of knowing and originating movement. As the primordial principle from which all other things are derived, it is cognitive; as finest in grain, it has the power to originate movement.

Heraclitus says that the first principle-the ‘warm exhalation’ of which, according to him, everything else is composed-is soul; further, that this exhalation is most incorporeal and in ceaseless flux; that what is in movement requires that what knows it should be in movement; and that all that is has its being essentially in movement.

Against the Notion that Soul is Self-Movement

Not only is it false that the essence of soul is what moves (or is capable of moving) itself, but it is an impossibility that movement should be even an attribute of it.

If there be a movement natural to the soul, there must be a counter-movement unnatural to it, and conversely. The same applies to rest as well as to movement; for the terminus ad quem of a thing’s movement is the place of its rest. But what meaning can be attached to enforced movements or rests of the soul, it is difficult even to imagine.

If the soul moves itself, it must be the mover itself that is moved, so that it follows that if movement is in every case a displacement of that which is in movement, in that respect in which it is said to be moved, the movement of the soul must be a departure from its essential nature, at least if its self-movement is essential to it, not incidental.

Movement is not in the soul, but that sometimes it terminates in the soul and sometimes starts from it, sensation e.g. coming from without inwards, and reminiscence starting from the soul and terminating with the movements, actual or residual, in the sense organs.

The case of mind is different; it seems to be an independent substance implanted within the soul and to be incapable of being destroyed. The incapacity of old age is due to an affection not of the soul but of its vehicle, as occurs in drunkenness or disease. Thus it is that in old age the activity of mind or intellectual apprehension declines only through the decay of some other inward part; mind itself is impassible. Thinking, loving, and hating are affections not of mind, but of that which has mind, so far as it has it. That is why, when this vehicle decays, memory and love cease; they were activities not of mind, but of the composite which has perished; mind is, no doubt, something more divine and impassible.

Criticism of Plato’s Timaeus

Plato in Timaeus implies that the movements of the soul are identified with the movements of the heavens. After compounding the soul-substance out of the elements and dividing it in accordance with the harmonic numbers, in order that it may possess a connate sensibility for ‘harmony’, the Demiurge bent the straight line into a circle; this single circle he divided into two circles united at two common points; one of these he subdivided into seven circles.

Aristotle: It is a mistake to say that the soul is a spatial magnitude. Now mind is one and continuous in the sense in which the process of thinking is so, and thinking is identical with the thoughts which are its parts; these have a serial unity like that of number, not a unity like that of a spatial magnitude.”

Nemo: The spacial magnitudes can all be described by numbers, and therefore possess numerical unity.

Aristotle: If mind were indeed a spatial magnitude, how could it possibly think? Will it think with any one indifferently of its parts? If contact of any part whatsoever of itself with the object is all that is required, why need mind move in a circle, or indeed possess magnitude at all? On the other hand, if contact with the whole circle is necessary, what meaning can be given to the contact of the parts?

Nemo: A point is a part of a circle, which is the whole. Mind cannot have knowledge of the whole without knowledge of all its parts, and vice versa; it cannot think of the whole without thinking of the parts at the same time. The magnitude of the circle, instead of a single point, is necessary, because thinking and knowing entail two things, one that knows and the other that is known, and if the two are the same, as in self-reflection, thinking becomes a circular movement.

Aristotle: If the circular movement is eternal, there must be something which mind is always thinking-what can this be? For all practical processes of thinking have limits-they all go on for the sake of something outside the process, and all theoretical processes come to a close in the same way as the phrases in speech which express processes and results of thinking. Every such linguistic phrase is either definitory or demonstrative. Demonstration has both a starting-point and may be said to end in a conclusion or inferred result; even if the process never reaches final completion, at any rate it never returns upon itself again to its starting-point.

Nemo: In the thinking process which is eternal, the immortal soul contemplates itself, as in a mirror, and it is all-encompassing. Though it seems to proceed in a linear manner in logical reasoning, it always returns to itself in self-reflection and recollection.

Aristotle: Some community of nature is presupposed by the fact that the one acts and the other is acted upon, the one moves and the other is moved; interaction always implies a special nature in the two interagents. It is absurd to think that any soul could be clothed upon with any body, as in the Pythagorean myths, for each body seems to have a form and shape of its own.

Nemo: All bodies consist of elementary particles, which constantly migrates/changes from one Form of body to another. Just as a potter can mould the same clay sometimes into one vessel sometimes into another, so the soul can be clothed sometimes in one body sometime another. While the soul is immortal, the body is mortal, and the mortal cannot contain the immortal.

Body is Potentiality, Soul is Actuality

Soul and mind are distinct, according to Aristotle. The former is mortal, whereas the latter is immortal.

Matter is potentiality, form actuality; of the latter there are two grades related to one another as e.g. knowledge to the exercise of knowledge.

The body is the subject or matter, not what is attributed to it. Hence the soul must be a substance in the sense of the form of a natural body having life potentially within it. But substance is actuality, and thus soul is the actuality of a body as above characterized. The body so described is a body which is organized. In other words, soul is an actuality or formulable essence of something that possesses a potentiality of being besouled.

Plants when divided are observed to continue to live, thus showing that in their case the soul of each individual plant before division was actually one, potentially many. We notice a similar result in other varieties of soul, i.e. in insects which have been cut in two; each of the segments possesses both sensation and local movement.

The soul is the cause or source of the living body. The terms cause and source have many senses. But the soul is the cause of its body alike in all three senses which we explicitly recognize. It is (a) the source or origin of movement, (b) the end, (c) the essence of the whole living body.

The Order of Living Things

The psychic powers are the nutritive, the appetitive, the sensory, the locomotive, and the power of thinking. Plants have none but the first, the nutritive, while another order of living things has this plus the sensory. If any order of living things has the sensory, it must also have the appetitive; for appetite is the genus of which desire, passion, and wish are the species; now all animals have one sense at least, viz. touch, and whatever has a sense has the capacity for pleasure and pain and therefore has pleasant and painful objects present to it, and wherever these are present, there is desire, for desire is just appetition of what is pleasant.

The cases of figure and soul are exactly parallel; for the particulars subsumed under the common name in both cases-figures and living beings-constitute a series, each successive term of which potentially contains its predecessor, e.g. the square the triangle, the sensory power the self-nutritive. They are related in this serial way. The power of perception is never found apart from the power of self-nutrition, while-in plants-the latter is found isolated from the former. Again, no sense is found apart from that of touch, while touch is found by itself; those which possess thought have all the other powers above mentioned, while the converse does not hold-indeed some live by imagination alone, while others have not even imagination. The mind that knows with immediate intuition presents a different problem.

1. The Nutritive Soul

Nutrition and reproduction are due to one and the same psychic power. For any living thing that has reached its normal development and which is unmutilated, and whose mode of generation is not spontaneous, the most natural act is the production of another like itself, an animal producing an animal, a plant a plant, in order that, as far as its nature allows, it may partake in the eternal and divine.

What serves as food to a living thing is what is contrary to it-not that in every pair of contraries each is food to the other: to be food a contrary must not only be transformable into the other and vice versa, it must also in so doing increase the quantity of the other. One set of thinkers assert that like fed, as well as increased in amount, by like. Another maintain the very reverse, viz. that what feeds and what is fed are contrary to one another and that like is incapable of being affected by like. If we use the word food of both, viz. of the completely undigested and the completely digested matter, we can justify both the rival accounts of it; taking food in the sense of undigested matter, it is the contrary of what is fed by it, taking it as digested it is like what is fed by it.

Food has a power which is other than the power to increase the bulk of what is fed by it; so far forth as what has soul in it is a quantum, food may increase its quantity, but it is only so far as what has soul in it is a ‘this-somewhat’ or substance that food acts as food; in that case it maintains the being of what is fed. Further, it is the agent in generation, i.e. not the generation of the individual fed but the reproduction of another like it.

2. The Sensitive Soul

Everything that is acted upon or moved is acted upon by an agent which is actually at work. Hence it is that in one sense, what acts and what is acted upon are like, in another unlike, i.e. prior to and during the change the two factors are unlike, after it like.

In the case of what is to possess sense, the first transition is due to the action of the male parent and takes place before birth so that at birth the living thing is, in respect of sensation, at the stage which corresponds to the possession of knowledge. Actual sensation corresponds to the stage of the exercise of knowledge. But between the two cases compared there is a difference; the objects that excite the sensory powers to activity are outside, and individuals, while what knowledge apprehends is universals, and these are in a sense within the soul. That is why a man can exercise his knowledge when he wishes, but his sensation does not depend upon himself a sensible object must be there.

What is the Power to Perceive

What perceives is, of course, a spatial magnitude, but we must not admit that either the having the power to perceive or the sense itself is a magnitude; what they are is a certain ratio or power in a magnitude. This enables us to explain why objects of sense which possess one of two opposite sensible qualities in a degree largely in excess of the other opposite destroy the organs of sense; if the movement set up by an object is too strong for the organ, the equipoise of contrary qualities in the organ, which just is its sensory power, is disturbed; it is precisely as concord and tone are destroyed by too violently twanging the strings of a lyre.

All sense-perception is a process of being so affected. That is why when an object of touch is equally hot and cold or hard and soft we cannot perceive; what we perceive must have a degree of the sensible quality lying beyond the neutral point. This implies that the sense itself is a ‘mean’ between any two opposite qualities which determine the field of that sense. It is to this that it owes its power of discerning the objects in that field.

Those without the power to perceive are so because they have no mean of contrary qualities, and so no principle in them capable of taking on the forms of sensible objects without their matter.

That which cognizes must have an element of potentiality in its being, and one of the contraries must be in it. But if there is anything that has no contrary, then it knows itself and is actually and possesses independent existence.


Is it the case then that what discriminates, though both numerically one and indivisible, is at the same time divided in its being? In one sense, it is what is divided that perceives two separate objects at once, but in another sense it does so qua undivided; for it is divisible in its being but spatially and numerically undivided. is not this impossible? For while it is true that what is self-identical and undivided may be both contraries at once potentially, it cannot be self-identical in its being-it must lose its unity by being put into activity. It is not possible to be at once white and black, and therefore it must also be impossible for a thing to be affected at one and the same moment by the forms of both, assuming it to be the case that sensation and thinking are properly so described.

The soul that discriminates one sense from another is a sort of unity, but as a connecting term. The two faculties it connects, being one by analogy and numerically, are each to each as the qualities discerned are to one another.


The soul is in a way all existing things; for existing things are either sensible or thinkable. Within the soul the faculties of knowledge and sensation are potentially these objects, the one what is knowable, the other what is sensible. Not the matter, but its form is present in the soul, as actualized sense and knowledge. Mind is the form of forms and sense the form of sensible things.


Further Reading:


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