Summa contra Gentiles3.87 Chapter LXXXVII. That the movement of a heavenly body is not the cause of our choosing by virtue of its soul moving us, as some say

We must observe, however, that Avicenna also (Metaph. x.) holds that the movements of the heavenly bodies are the causes of our choice, not merely by being the occasion thereof, but even as a per se cause. For he holds the heavenly bodies to be animate: and, since the heaven’s movement proceeds from its soul, and is the movement of a body, it follows that just as forasmuch as it is a body’s movement, it must have the power to transform bodies, so, forasmuch as it comes from a soul, it must have the power to make impressions on our soul; wherefore the heavenly movement is the cause of our acts of will and choice. The position of Albumasar would seem to come to the same as expounded in the First Book of his Introductorium.

But this position is unreasonable. Because any effect that is caused by an agent through an instrument, must be proportionate to the instrument as well as to the agent: for we do not employ any instrument for any effect. Consequently it is not possible to produce by means of an instrument an effect which is utterly outside the scope of its action.

Now it is altogether beyond the scope of a body’s action to impress the intellect or will, as proved above; except perhaps indirectly by making an impression on the body, as we have said. Therefore it is impossible for a heavenly body’s soul, if it have one, to make an impression on the intellect and will by means of the movement of that heavenly body.

Moreover. The particular active cause, while acting, bears a resemblance to the universal active cause, and imitates it. Now if a human soul were to make an impression on another human soul through an action of the body, as when it reveals its mind by means of vocal signs, the bodily action that proceeds from the one soul does not reach the other soul except by means of the body: for the vocal sounds play on the organ of hearing, and thus being perceived by the sense, its meaning reaches the understanding. Consequently if the celestial soul makes an impression on our soul by means of a corporeal movement, its action will not reach our soul except through a change effected in our body. But this does not cause our choice, but only occasions it, as we have shown above. Therefore the heavenly movement is not the cause but only the occasion of our choice.

Again. Since mover and moved must be simultaneous, as is proved in 7 Phys. ii., it follows that movement must come from the first mover to the last thing moved in a certain order, so that, to wit, the mover moves that which is distant through that which is nearest. Now our body is nearer to the heavenly body which is supposed to be moved by the soul united to it, than our soul which is not ordered to the heavenly body except through its own body. This is proved by the fact that separate intellects are not ordered to a heavenly body, except perhaps as a mover to that which it moves. Therefore the impression of a heavenly body that originates in its soul does not reach our soul save through our body. And our soul is not moved in response to the movement of the body except accidentally, nor does choice result from an impression made on the body except as occasioned thereby, hi. ii as we have said. Therefore the heavenly body’s movement cannot be the cause of our choice through coming from its soul.

Besides. According to the opinion of Avicenna and certain other philosophers, the active intellect is a separate substance which acts on our souls in so far as it makes what is potentially intelligible to be understood actually. Now this is the result of abstraction from all material conditions, as is clear from what we have said in the Second Book. Consequently that which acts directly on the soul, does so not by means of a corporeal movement, but rather by abstraction of everything corporeal. Therefore the soul of heaven, if it have a soul, cannot be through the heavenly movement the cause of our acts of choosing or understanding.

By the same arguments it can be proved that the heavenly movement is not the cause of our choice by the power of a separate substance, if anyone suppose the heaven not to be animate, but to be moved by a separate substance.