“Parva Naturalia” by Aristotle

In this collection of treatises, Aristotle employs the scientific method (namely, observation, inference, hypothesis and empirical proof) to determine the nature and cause of the joint activities of body and soul, namely, sense perception, memory, sleep, dreams, breathing, aging and death.

The Final Cause of Senses

Touch and taste necessarily appertain to all animals, touch, for the reason given in On the Soul, and taste, because of nutrition. It is by taste that one distinguishes in food the pleasant from the unpleasant, so as to flee from the latter and pursue the former: and savour in general is an affection of nutrient matter. The senses which operate through external media, viz. smelling, hearing, seeing, are found in all animals which possess the faculty of locomotion. To all that possess them they are a means of preservation: guided by antecedent perception, they may both pursue their food, and shun things that are bad or destructive. But in animals which have also intelligence they serve for the attainment of a higher perfection. It is hearing that contributes most to the growth of intelligence. For rational discourse is a cause of instruction in virtue of its being audible, which it is, not directly, but indirectly; since it is composed of words, and each word is a thought-symbol. Accordingly, of persons destitute from birth of either sense, the blind are more intelligent than the deaf and dumb.

Sense and the Sensible

The sensory organ is only potentially that which the sense, as realized, is actually; since the object of sense is what causes the actualization of each sense, so that it (the sense) must (at the instant of actualization) be (actually) that which before (the moment of actualization) it was potentially.

[–Update in 2014–]

The activity of sense-perception in general is analogous, not to the process of acquiring knowledge, but to that of exercising knowledge already acquired.

Any two things can affect, or be affected by, one another only so far as contrariety to the other resides in either of them.

1. Color

Light is the colour of the Translucent incidentally. Translucent, as we call it, is not something peculiar to air, or water, but is a common ‘nature’ and power, capable of no separate existence of its own, but subsisting in all bodies in a greater or less degree. As the bodies in which it subsists must have some extreme bounding surface, so too must this. Light is a ‘nature’ inhering in the Translucent when the latter is without determinate boundary. But when the Translucent is in determinate bodies, its bounding extreme is colour. 1

2. Savour

As persons washing Colours or Savours in a liquid cause the water in which they wash to acquire such a quality [as that of the colour or savour], so nature, too, by washing the Dry and Earthy in the Moist, and by filtering the latter, that is, moving it on by the agency of heat through the dry and earthy, imparts to it a certain quality. This affection, wrought by the aforesaid Dry in the Moist, capable of transforming the sense of Taste from potentiality to actuality, is Savour. Savour brings into actual exercise the perceptive faculty which pre-existed only in potency.

No single element, but only composite substance, constitutes nutriment for animals. It is qua hot or cold that the food assimilated causes these; for the heat or cold is the direct cause of growth or decay. It is qua gustable, however, that the assimilated food supplies nutrition. For all organisms are nourished by the Sweet [i.e. the ‘gustable’ proper], either by itself or in combination with other savours.

3. Odour

The nature of Odours must be analogous to that of Savours; inasmuch as the Sapid Dry effects in air and water alike, but in a different province of sense, precisely what the Dry effects in the Moist of water only.

One class of odours is that which runs parallel to savours: to odours of this class their pleasantness or unpleasantness belongs incidentally; The other class of odours consists of those agreeable in their essential nature, e.g. those of flowers. For these do not in any degree stimulate animals to food, nor do they contribute in any way to appetite. The reason why the perception of the latter is peculiar to man is found in the characteristic state of man’s brain. For his brain is naturally cold, and the blood which it contains in its vessels is thin and pure but easily cooled (whence it happens that the exhalation arising from food, being cooled by the coldness of this region, produces unhealthy rheums); therefore it is that odours of such a species have been generated for human beings, as a safeguard to health.

4. Sight

Stimulatory motion is set up even by slight differences, and sense-perception is quick to respond to it; The organ which perceives colour is not only affected by its object, but also reacts upon it. As the eye [in seeing] is affected [by the object seen], so also it produces a certain effect upon it. If a woman chances during her menstrual period to look into a highly polished mirror, the surface of it will grow cloudy with a blood-coloured haze.

Democritus’ Atomic Theory of the Sense Perception

Democritus represents all objects of sense as objects of Touch, with atomic figures such as Magnitude and Figure, Roughness and Smoothness, Sharpness and Bluntness.

The Controlling Sensory Faculty

Every sense has something peculiar, and also something common in virtue whereof a person perceives that he sees or hears (for it is not by mere taste, or sight, or both together that one discerns, and has the faculty of discerning, that sweet things are different from white things, but by a faculty connected in common with all the organs of sense; for there is one sensory function, and the controlling sensory faculty is one, though differing as a faculty of perception in relation to each genus of sensibles).

This common sensory activity subsists in association chiefly with the faculty of touch (for this can exist apart from all the other organs of sense, but none of them can exist apart from it); It is therefore evident that waking and sleeping are an affection of this [common and controlling organ of sense-perception]. This explains why they belong to all animals, for touch alone is common to all animals.

Sense-perception in animals originates ill the same part of the organism in which movement originates.

Memory and Recollection

One must cognize magnitude and motion by means of the same faculty by which one cognizes time (i.e. by that which is also the faculty of memory), and the presentation (involved in such cognition) is an affection of the sensus communis, i.e. the primary faculty of perception.
Memory or remembering is the state of a presentation, related as a likeness to that of which it is a presentation.

Acts of recollection, as they occur in experience, are due to the fact that one movement (of the mind) has by nature another that succeeds it in regular or customary order.

For the purpose of recollection, one should cognize, determinately or indeterminately, the time-relation (of that “which he wishes to recollect). There is something by which one distinguishes a greater and a smaller time; and it is reasonable to think that one does this in a way analogous to that in which one discerns (spacial) magnitudes. When the ‘movement’ corresponding to the object and that corresponding to its time concur, then one actually remembers.


Sleep is not co-extensive with any and every impotence of the perceptive faculty, but this affection is one which arises from the evaporation attendant upon the process of nutrition.

In every animal the hot naturally tends to move [and carry other things] upwards, but when it has reached the parts above, e.g. the brain is the coolest part of the body, [becoming cool], it turns back again, and moves downwards in a mass. When this comes to a stand it weighs a person down and causes him to nod, but when it has actually sunk downwards, and by its return has repulsed the hot, sleep comes on, and the animal so affected is presently asleep. Owing to the fact that the blood formed after the assimilation of food is especially in need of separation, sleep [then especially] occurs [and lasts] until the purest part of this blood has been separated off into the upper parts of the body (i.e., the head), and the most turbid into the lower parts. When this has taken place animals awake from sleep, being released from the heaviness consequent on taking food.

The cause of sleeping consists in the recoil by the corporeal element, upborne by the connatural heat, in a mass upon the primary sense-organ; sleep is a seizure of the primary sense-organ, rendering it unable to actualize its powers; arising of necessity, i.e. for the sake of its conservation; since remission of movement tends to the conservation of animals.

Interpreting Dreams

Are we then to say that some dreams are causes, others tokens, e.g. of events taking place in the bodily organism? … For the movements which occur in the daytime [within the body] are, unless very great and violent, lost sight of in contrast with the waking movements, which are more impressive. In sleep the opposite takes place, for then even trifling movements seem considerable. This is plain in what often happens during sleep; for example, dreamers fancy that they are affected by thunder and lightning, when in fact there are only faint ringings in their ears;…. When they are awakened, these things appear to them in this their true character. But since the beginnings of all events are small, so, it is clear, are those also of the diseases or other affections about to occur in our bodies. In conclusion, it is manifest that these beginnings must be more evident in sleeping than in waking moments.

The most skillful interpreter of dreams is he who has the faculty of observing resemblances. Dream presentations are analogous to the forms reflected in water. In the latter case, if the motion in the water be great, the reflexion has no resemblance to its original, nor do the forms resemble the real objects. Accordingly, in the other case also, in a similar way, some such thing as this [blurred image] is all that a dream amounts to; for the internal movement effaces the clearness of the dream.

Life and Respiration

Genesis from seeds always starts from the middle. All seeds are bivalvular, and the place of junction is situated at the point of attachment (to the plant), an intermediate part belonging to both halves.

Likewise in sanguineous animals the heart is the first organ developed; It is from the heart that the veins issue, and that in sanguineous animals the blood is the final nutriment from which the members are formed. Hence in sanguineous animals the source both of the sensitive and of the nutritive soul must be in the heart, for the functions relative to nutrition exercised by the other parts are ancillary to the activity of the heart.

The universal cause of the need which the animal has for refrigeration, is the union of the soul with fire that takes place in the heart. Respiration is the means of effecting refrigeration.

Life and the presence of soul involve a certain heat. Not even the digesting process to which is due the nutrition of animals occurs apart from soul and warmth, for it is to fire that in all cases elaboration is due. It is for this reason, precisely, that the primary nutritive soul also must be located in that part of the body and in that division of this region which is the immediate vehicle of this principle.

We must not entertain the notion that it is for purposes of nutrition that respiration is designed, and believe that the internal fire is fed by the breath; respiration, as it were, adding fuel to the fire, while the feeding of the flame results in the outward passage of the breath. To combat this doctrine I shall repeat what I said in opposition to the previous theories. This, or something analogous to it, should occur in the other animals also (on this theory), for all possess vital heat. Further, how are we to describe this fictitious process of the generation of heat from the breath? Observation shows rather that it is a product of the food. A consequence also of this theory is that the nutriment would enter and the refuse be discharged by the same channel, but this does not appear to occur in the other instances.

No animal yet has been seen to possess both lungs and gills, and the reason for this is that the lung is designed for the purpose of refrigeration by means of the air (it seems to have derived its name (pneumon) from its function as a receptacle of the breath (pneuma)), while gills are relevant to refrigeration by water. Now for one purpose one organ is adapted and one single means of refrigeration is sufficient in every case.

Aging and Death

Generation is the initial participation, mediated by warm substance, in the nutritive soul, and life is the “maintenance of this participation. Youth is the period of the growth of the primary organ of refrigeration, old age of its decay, while the intervening time is the prime of life.

A violent death or dissolution consists in the extinction or exhaustion of the vital heat (for either of these may cause dissolution), while natural death is the exhaustion of the heat owing to lapse of time, and occurring at the end of life. In plants this is to wither, in animals to die. Death, in old age, is the exhaustion due to inability on the part of the organ, owing to old age, to produce refrigeration.

The Error of Other Natural Philosophers

The main reason why these writers have not given a good account of these facts is that they did not accept the doctrine that there is a final cause for whatever Nature does. If they had asked for what purpose respiration exists in animals, and had considered this with reference to the organs, they would have discovered the reason more speedily.


1^: Compare with Aristotle’s exposition of the senses in his treatise “On the Soul”:

Whatever is visible is colour and colour is what lies upon what contains in itself the cause of visibility. Every colour has in it the power to set in movement what is actually transparent; that power constitutes its very nature. Colour sets in movement not the sense organ but what is transparent, e.g. the air, and that, extending continuously from the object to the organ, sets the latter in movement. Neither air nor water is transparent because it is air or water; they are transparent because each of them has contained in it a certain substance which is the same in both and is also found in the eternal body which constitutes the uppermost shell of the physical Cosmos. Of this substance light is the activity-the activity of what is transparent so far forth as the latter has in it the determinate power of becoming transparent; where this power is present, there is also the potentiality of the contrary, viz. darkness. Light is as it were the proper colour of what is transparent, and exists whenever the potentially transparent is excited to actuality by the influence of fire or something resembling ‘the uppermost body’; for fire too contains something which is one and the same with the substance in question. Light is certainly not a body, for two bodies cannot be present in the same place.

What has the power of producing sound is what has the power of setting in movement a single mass of air which is continuous from the impinging body up to the organ of hearing. The organ of hearing is physically united with air, and because it is in air, the air inside is moved concurrently with the air outside. Air in itself is, owing to its friability, quite soundless; only when its dissipation is prevented is its movement sound. The air in the ear is built into a chamber just to prevent this dissipating movement, in order that the animal may accurately apprehend all varieties of the movements of the air outside. That is why we say that we hear with what is empty and echoes, viz. because what we hear with is a chamber which contains a bounded mass of air.

While in respect of all the other senses we fall below many species of animals, in respect of touch we far excel all other species in exactness of discrimination. That is why man is the most intelligent of all animals. This is confirmed by the fact that it is to differences in the organ of touch and to nothing else that the differences between man and man in respect of natural endowment are due; men whose flesh is hard are ill-endowed by nature, men whose flesh is soft, well endowed.

We do perceive everything through a medium; flesh is not the organ but the medium of touch.



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