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Whether operations occur through similar or dissimilar parts.

Further one inquires whether operations occur through similar or dissimilar parts.

And it seems first with respect to the nutritive power that they occur through similar parts. For certain animals in which organic parts are not present have a vegetative soul. Therefore, their operation will be through similar parts.

The same seems true with respect to the vital power, since certain beings live which are lacking in organic parts, and therefore, etc.

The same seems true with respect to the sensitive power. For one element is predominant in an organ of any sensitive power whatsoever, just as water is predominant in the organ of sight, etc. But that one in which one element predominates is a similar part. Therefore, the sensitive power operates through similar parts.

Moreover, according to Aristotle in the second book of On the Soul, the nature of a medium and of an organ is one and the same. But the medium in the senses is similar to that in the parts; therefore, also the organ will be similar.

The same seems true with respect to the motive power. Now in any thing, whenever dilation occurs on account of the appearance of something delightful, or contraction occurs on account of the appearance of something noxious, then in that same thing a motive power must be present. But these occur in similar parts, and therefore, etc.

One argues the opposite, first with respect to the vegetative power. For a vegetative operation occurs through that without which the vegetative operation cannot be completed. But a vegetative operation is not completed without dissimilar parts. This is clear in those that have blood, for blood only nourishes if it is digested in the liver and then carried to the individual members through the veins. Therefore, the nutritive power operates through dissimilar parts.

Second, the same thing seems true with respect to the vital power, which is in the heart, which the arteries, the lungs, and the homogenous members serve. But these are official members, and therefore, etc.

Third, the same thing seems true for the sensitive power, because for vision a pupil is required, or the glacial humor and the hollow nerve. And members like these are dissimilar, and therefore, etc.

The same thing seems true for the motive power, since, according to the Philosopher in the third book of On the Soul, in the first organ of motion there exist both something moving and something resting. And everything like this in the parts is dissimilar, and therefore, etc.

To this question one should respond that some operations are due to homogenous parts and some to dissimilar parts. Now the operations of the nutritive power and the sensitive power are first due to similar parts, but the operations of the vital power and the motive power are due to dissimilar parts. For the natural power's operations are: to attract, digest, retain, and expel, and each of these occurs through some homogenous part. Now although these occur through many parts, because flesh attracts and bone attracts as do vein and nerve, nevertheless each part taken per se is homogenous. And this is why we say that natural operations first occur through homogenous parts, and secondarily through dissimilar parts that are made up of similar ones.

It is the same for the sensitive operation. For touch is the first sense, and touch is in every part of the body according to Avicenna, who says that God armed the animal everywhere with touch lest it suffer some ill unawares. But nothing occurs in every part unless it is a homogenous part, and this is why the operation of touch occurs first through a homogenous part. In the same way, various parts come together for the operations of the other senses. Consider that in sight the glacial humor receives the appearance of a thing [ species] and the hollow nerve remands it to the first visual principle. But these parts, taken individually, are homogenous, and this is why the sensitive power universally operates through similar parts first. Nevertheless, the operation of the external senses is only completed by means of dissimilar parts.

But the vital power exists principally in the heart; however, the heart is a dissimilar part, and other parts serve it. For the heart infuses the vital power, and the other parts receive it. Thus, in those breathing or having blood it is clear that the vital power is kept safe in the heart only if the heat of the heart is tempered by the reception of something extrinsic to it. And this is why animals that walk breathe, and breathing occurs only by means of a dissimilar part.

But it is clear regarding the motive power that wherever there is a motive power, there is something moving and something moved, and these are dissimilar.

Or one can put it another way. For all operations occur through similar parts and dissimilar parts, but some occur first through similar parts and only secondarily through dissimilar ones, and some do the opposite.

As a result, the arguments conclude as true in part.[1]

  • [1] One possible translation of these cryptic words: Unde rationes pro parte verum concludunt.
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