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Whether sleep is an affect [passio] of the heart or the brain.

Further one asks whether sleep is an affect of the heart or the brain.

1. It seems it is an affect of the heart. For the same thing is the principle of motion and rest. But the heart is the principle of motion, and therefore also of rest and, consequently, of sleep.

2. Furthermore, motion follows sense. Therefore, when sense is restrained, motion is prevented. But when motion is prevented, an impediment occurs or its principle is impeded, and this is the heart; therefore, etc.

3. Moreover, during sleep heat and the spirit turn back to the interior parts. Therefore, sleep is an affect of that part from which the heat and the spirit flow. But this part is the heart; therefore, etc.

To the contrary. Sleep is not an affect of that part whose power is not restrained during sleep. But the heart is not restrained in sleep, for it infuses heat and spirit and life, which is evident from the pulse that moves during sleep just as it does during waking.

Moreover, sleep is a fetter for sensation as far as its operation is concerned. But the brain is the principle of sensation so far as its operation is concerned. Therefore, sleep is an affect of the brain.

One must respond that sleep is an affect of the brain. For sleep is caused by a multiplying of the vapor all the way to the brain. For when the vapors that have risen are condensed by the cold and moisture of the brain, the external senses are clouded and the animal sleeps. And this is why animals with small brains, like flyers, or with dry brains, like the dog, sleep less than a human, whose brain is large, cold, and moist. Thus in a small brain, as in a flyer, the vapors are not so condensed, and this is why it does not sleep as much. And, once again, children sleep more than old people owing to the abundance of moisture that is in children. It is clear from this that the brain is the principle of sense and motion with respect to their operation. This is so even though the heart is their principle with respect to their first origin, and during sleep sense and motion are restrained with respect to their operations and not with respect to their first act, in which respect they are referred to the heart. This then is a sign that sleep is an affect of the brain and not of the heart.

1. On to the arguments. To the first, one must say that the principle of sense is threefold. The first exists with respect to its origin; the second, with respect to its operation; and the third with respect to its turning back internally. The heart is the principle in the first and third modes. The brain is the principle in the second, and this is why, just as the brain is the principle of sensitive motion, so too is it the principle of rest. And this rest is sleep, because opposites have to concern the same thing.

2. To the second, one must say that the heart is the immediate principle of heat and of the spirits, but not of sensitive motion, and this is why, although sensitive motion may be impeded, it is not necessary that the heart be impeded.

3. To the third argument one must reply that heat exists for the sake of three things: namely, for life, for nourishment, and for the operations of the senses, because without heat there can be no operation of sense. And during sleep a turning-back of heat occurs with respect to the third instance, namely, the heat of the sensitive operations, so much so that it abandons the external senses altogether, but does not do so in the first and second instances. And this is why it is not necessary for sleep to be an affect of the heart.

Whether sleep is an affect of the common or a particular sense.

Further one asks of which power is sleep an affect [ passio], namely, of the common sense or of a particular sense.

1. And it seems to be an affect of a particular sense. For sleep is a restraint and an affect of the first sense, as On Sleep and

Waking maintains.[1] But the first sense is touch; therefore, etc.

2. Moreover, all the members are restrained during sleep. Therefore, it belongs to that power that extends through all the members. But such is touch. Therefore, etc.

3. Moreover, sleep is the result of that power whose nature is to be acted on by things that cause sleep.[2] But sleep is caused by the action of sensible qualities. Therefore, it is an affect [passio] of touch since qualities are the object of touch.

4. Besides, an interior power is strengthened during sleep, and an external power is weakened. But the common sense is an internal power. Therefore, sleep is not its affect, but rather the affect of something external.

The Philosopher suggests the contrary in On Sleep and Waking, for he says that "sleep fetters the first sensitive." But the first sensitive is the common sense; therefore, etc.

It must be said that sleep is an affect [passio] of the common sense. For no one particular sense is restrained during sleep, but each one is. It is then necessary that something be restrained first on which all the particular senses are based, and such is the common sense. For common sense is related to the particular senses just as the center is related to the lines stretching out to the circumference, as the Philosopher has it in the second book of On the Soul. The sensitive power is diffused by the common sense to each of the particular senses, and all the variations of all the senses are terminated at that one, and this is why the particular senses are restrained when the common sense is restrained.

Moreover, sleep is an affect of the brain, as has been said. Therefore, it is an affect of some power whose organ is the brain. But the brain is not an organ of one particular sense.

1. On to the arguments. To the first, one must reply that something can be called a principle with respect to something else in four ways: in one way, with reference to its origin; in another way, with respect to the immediate influence of a power; in a third way with respect to a necessity of life; and in a fourth way with respect to cognition. In the first way the vital power is a principle of sense; in the second way, the common sense is a principle of the particular senses; in the third way, touch is the principle of the other senses; but in the fourth way it is sight, because it reveals to us the many differences among things, as the Philosopher says in the first book of the Metaphysics. Thus when the Philosopher says that "sleep is the affect of the first sensitive," one must say that he is speaking of the first sensitive in the second way, and not in the third way.

2. To the second argument one must reply that during sleep restraint can exist in individual members in two ways: either because that which is diffused throughout all the members is restrained first, or because something is restrained earlier because it is diffused in this way. Thus, in sleep, there is a stupor in all the members, not because touch is restrained first but because the common sense, from which the power of touch flows, is restrained.

3. To the third argument one must reply that something that is acted on [ pati] by those things that cause sleep can be acted on in two ways: either through a corporeal impression alone, or with the judgment [discretion of such an impression. Touch is thus acted on by tangible objects, because it judges among them. But the common sense is not acted on during sleep in this way; rather, the organ of the common sense receives a corporeal impression from the tangibles themselves, and that is why its power is dulled. Thus, although sleep is caused by the action of tangibles, nevertheless it does not belong to its affect [passio] <neither is it the result of its power>[3] that it is acted on by them with judgment, but belongs to that which is acted on through the mediation of a corporeal impression.

4. To the last, one must respond that an interior power is multiple: one is natural, vital, and another animal. In sleep the vital power is strengthened, to wit the natural power of the heart and the lungs and the like. An indication of this is that digestion is performed better during sleep than during waking. This is why sweat is expelled more easily in sleep than while awake, because sweat arises from the work of the natural power. But the interior animal power is not strengthened [during sleep]. Still it can be said that one interior animal power is only receptive, like common sense, and another is retentive, like the estimative and imaginative. And in sleep the interior sense is restrained with respect to apprehension but not with respect to the retention of things apprehended earlier, and not with respect to the comparison of these things to one another. And this is why many people while sleeping imagine many things which they do not imagine while awake, and they recollect and dream. Nevertheless the interior sense is sometimes restrained absolutely with respect to all its powers and then nothing whatsoever appears to the one asleep.

  • [1] Ar., De somno et vigilia 2 (455bio); 3 (458a28-2Q). Cf. A., De somno et vigilia 1.2.1.
  • [2] We have emended the unintelligible virtutis cuius natum to read virtutis cuius natura.
  • [3] The material within <> represents the editor's addition.
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