LESSON 9

The Number of Primary Movers

  Chapter 8: 1073a 14-1073b 17

             1078. We must not neglect the question whether it is necessary to posit one such substance or more than one, and if the latter, how many; and we must also recall the lack of statements on this point by other philosophers, because they have said nothing about the number of these substances which can be clearly stated. The theory of Ideas makes no proper study of this problem; for the proponents of the Ideas say that the Ideas are numbers, and they speak of numbers sometimes as unlimited and sometimes as limited to the number ten. But as to the reason why there should be so many numbers, nothing is said apodictically.

             1079. However, we must discuss this question by beginning with what has already been laid down and established. For the first principle and primary being is both essentially and accidentally immovable, but it causes the primary motion, which is eternal and unique. And since that which is moved must be moved by something else, the first mover must be essentially immovable, and eternal motion must be caused by an eternal mover, and a single motion by a single thing.

             1080. Now we see that, besides the simple local motion which we say the first immovable substance causes, there are other local motions--those of the planets--which are eternal (for a body which is moved in a circle is eternal and never stands still, as has been proved in our treatises on nature . Each of these motions, then, must also be caused by a substance which is essentially immovable and eternal. For the nature of the stars is eternal, being a kind of substance; and that which causes motion is eternal and prior to that which is moved; and that which is prior to a substance must be a substance. Hence it is evident that there must be as many substances as there are motions of the stars, and that these substances are eternal in nature, essentially immovable, and without magnitude, for the reason given above (1076). It is evident, then, that these movers are substances, and that one of these is first and another second according to the same order as the motions of the stars.

             1081. But it is now necessary to discover the number of these motions from that branch of the mathematical sciences which is most akin to philosophy, namely, astronomy. For this science studies the kind of substance which is sensible but eternal, whereas the other mathematical sciences, such as the science of numbers and geometry, are not concerned with any kind of substance. That there are many motions belonging to the bodies which are moved is evident even to those who have given little consideration to the matter; for each of the wandering stars has more than one motion. As to the number of these motions, in order that we may have some definite number in mind for the purpose of understanding this point, let us now state what some of the mathematicians say; but for the rest, this we must investigate partly for ourselves and partly accept the opinion of other investigators. And if anyone in treating this subject should be found to form a different opinion from the one stated here, we must respect both views but accept the more certain.

COMMENTARY

             2553. Having shown what it is that constitutes the perfection of an immaterial substance, here the Philosopher asks whether this substance is one or many; and in regard to this he does three things. First (1078:C 2553), he indicates that it is necessary to treat this question because nothing definite has been said about it by other thinkers. Second (1079:C 2555), he shows that there are many such substances ("However, we must discuss"). Third (1081:C 2563), he shows how many there are ("But it is now necessary").

             He accordingly says, first (1078), that we must not neglect the question whether it is necessary to posit only one such substance which is eternal and immaterial or many; and if the latter, how many. But we must also "recall the lack of statements on this point by other philosophers," i.e., the fact that others have said nothing that is clear and evident about the number of these substances.

             2554. This is made clear as follows. Those who made a special claim for immaterial substances were the proponents of the Ideas. Now the opinion about the nature of the Ideas contains no theory about any definite number, because there are assumed to be Ideas of all things which share in a common name. But since those who posited Ideas said that they are numbers, it would seem that we could get some notion about how many numbers there are. However, they did not always say the same thing on this point. Sometimes they said that the species of numbers are unlimited. This is true of numbers by reason of their proper nature, because whenever a unit is added it always produces a different species of number. Hence, since in the case of numbers infinite additions can be made, the species of numbers may increase to infinity. At other times they said that the species of numbers are limited to the number ten. This refers to the naming of numbers, for the names of all numbers after ten seem to repeat in some way the name of a primary number. But they cannot show by any definite argument why there should be just so many numbers, i.e., ten, and not more or fewer. Nor is this to be wondered at, since this limitation of the species of numbers is not a real limitation but a nominal one. Other thinkers offer the argument that the number ten is generated from the progression of numbers up to the number four, which is the first square number. For one plus two equals three; and when three is added to this, the number six results; and when four is added to this, the number ten results.

             2555. However, we must discuss (1079).

             He now shows that there must be many substances of this kind; and in regard to this he does two things. First, he returns to the points established about the first principle. He says that, since other thinkers have said nothing demonstrative about the number of separate substances, we must discuss this question by beginning with what has already been laid down and established. For it has been said above that, while the first principle of beings is one which is neither essentially nor accidentally moved, it still causes a single motion, which is the first and eternal motion. For since everything which is moved must be moved by something else, as has been shown in Book VIII of the Physics, the first mover must be altogether immovable, and eternal motion must be caused by an eternal mover, and a single motion by a single mover.

             2556. Now we see (1080).

             Second, he shows that after the first principle it is necessary to posit a number of eternal substances. He says that besides the simple local motion of the universe (one that lasts a day--during which the entire heavens revolve--and is uniform and the most simple), which the first immovable substance causes, we observe the local motions of the planets, which are also eternal; because the circular body, i.e., a heaven, is also eternal. Therefore the eternity of motion is not destroyed as a result of the destruction of a movable being. And "it never stands still," i.e., it is incapable of coming to rest. Hence this motion is not broken by rest. These points have been proved in the philosophy of nature, both in the Physics as well as in The Heavens. Each of these motions, then, must be caused by a mover which is essentially unmoved and an eternal substance.

             2557. Now this must be so because the stars are eternal and are substances. Hence their mover must also be eternal and a substance; for a mover is prior to the thing moved, and that which is prior to a substance must be a substance. It is clear, then, that there must be as many substances as there are motions of the stars, and that these substances must be by nature eternal and essentially immovable and without magnitude, for the reason given above (1076:C 2548-50), i.e., because they move in infinite time and therefore have infinite power. Hence it is evident that there are immaterial substances which are as numerous as the motions of the stars, and that they also have the same order as the motions of the stars.

             2558. Now it must be borne in mind that after the first motion Aristotle computes only the motions of the planets, because at his time the motion of the fixed stars had not been detected. Hence he thought that the eighth sphere, in which the fixed stars are located, was the first one to be moved, and that its mover was the first principle. But later on astronomers perceived that the motion of the fixed stars was in an opposite direction to the first motion, so that above the sphere of the fixed stars it was necessary to posit another sphere, which surrounds the entire heavens and turns the whole in its daily motion. This is the first sphere, which is moved by the first mover of which Aristotle spoke.

             2559. But Avicenna claimed that the first sphere is moved directly, not by the first principle, but by an intelligence which is caused by the first principle. For since the first mover is absolutely one, Avicenna thought that only one thing could be caused by it; and this is the first intelligence, in which a plurality of potentiality and actuality is found inasmuch as it derives being from the first principle. For it is related to that on which it depends for its existence as something potential to something actual. Hence the first intelligence can immediately cause many things; for inasmuch as it understands itself as having some potentiality, it causes the substance of the orb which it moves, but insofar as it understands itself as possessing actual existence from some other cause, it causes the soul of its orb. Again, inasmuch as it understands its own principle, it causes the next intelligence, which moves a lower orb, and so on down to the sphere of the moon.

             2560. But this is not necessary. For an efficient cause in the realm of superior substances does not act like an efficient cause in the realm of material things, in the sense that a single effect is produced by a single cause, because among higher substances cause and thing caused have intelligible existence. Hence insofar as many things can be understood by a single superior substance, many effects can be produced by a single superior substance. And it seems quite fitting that the first motion of corporeal things, on which all other motions depend, should have as its cause the principle of immaterial substances, so that there should be some connection and order between sensible and intelligible things. A problem can arise, however, regarding the Philosopher's statement that the order of separate substances corresponds to the order of motions and bodies moved. For of all the planets the sun is the largest in size, and its effect is more evident in lower bodies; and even the motions of the other planets are arranged in accordance with the motion of the sun, and in a sense are subsequent to it. Hence it seems that the substance which moves the sun is nobler than the substances which move the other planets, even though the sun is not located above the other planets. But since among bodies one which contains is more formal, and is thereby nobler and more perfect, and is related to a contained body as a whole to a part, as is said in Book IV of the Physics; and since the sphere of a superior planet contains that of an inferior planet, therefore a superior planet, to which its whole sphere is subordinated, must have a higher and more universal power than an inferior planet, and must produce more lasting effects because it is nearer to the first sphere, which by its motion causes the eternality of things, as has been pointed out above (1065:C 2510). And this is the reason, as Ptolemy says in the Quadripartitum, why the effects of Saturn correspond to universal places and times, and those of Jupiter to years, and those of Mars, the sun, Venus and Mercury to months, and those of the moon to days.

             2561. This is also the reason why the effects of the planets appear in lower bodies in accordance with the order among the planets. For the first three highest planets seem to be directed to effects which pertain to the existence of a thing taken in itself; for the very stability of a thing's act of being is attributed to Saturn, and its perfection and state of well-being to Jupiter, and the power by which it protects itself from what is harmful and drives it away, to Mars. The other three planets seem to have as their proper effects the motion of a being. The sun is a universal principle of motion, and for this reason its operation is most evident in the case of lower motions. For Venus seems to have as its proper effect a more limited one, namely, the process of generation, by which a thing attains its form, and one to which all the other motions among lower bodies are directed. Mercury seems to have as its proper effect the multiplication of things, i.e., the distinction of individuals in the same species; and for this reason it has various motions. It is also mixed with the natures of all the planets, as the astronomers say. The changing of matter and the disposing of it to receive all celestial impressions belongs properly to the moon; and for this reason it seems that it is the planet which transmits celestial impressions and applies them to inferior matter.

             2562. Hence the higher a celestial body, the more universal, lasting, and powerful its effect. And since the celestial bodies are the instruments, so to speak, of the separate substances which cause motion, it follows that a substance which moves a higher orb has a more universal knowledge and power, and must therefore be nobler.

             2563. But it is now necessary (1081).

             Then he investigates the number of these substances; and this is divided into two parts. In the first part (1081:C 2563) he first investigates the number of celestial motions; and in the second (1084:C 2586), he infers from this the number of substances which cause motion ("Hence it is reasonable").

             In regard to the first he does two things. First, he indicates the source from which we must derive the number of celestial motions. Second (1082:C 2567), he gives the different opinions about this ("Now Eudoxus").

             He says, first (1081), that we must use the science of astronomy in studying the number of revolutions or celestial motions, which is a subject that belongs particularly to this branch of the mathematical sciences. For of these sciences only astronomy speculates about sensible and eternal substances, i.e., celestial bodies. But the other mathematical sciences do not consider any substance, as is clear in the case of arithmetic, which treats of numbers, and in the case of geometry, which treats of continuous quantity. Number and continuous quantity are accidents.

             2564. That there are many motions belonging to the bodies which move about in the heavens, i.e., the planets, is evident even to those who have little acquaintance with the science of astronomy; for "each of the wandering stars," i.e., the planets, is moved by several motions and not just by one. Now the planets are called "wandering stars," not because their motions are irregular, but because they do not always maintain the same pattern and position in relation to the other stars, as these do among themselves and for this reason are called "fixed."

             2565. That there are many motions of stars of this kind is detected in three ways. There is one motion which is perceived by plain sight. There is another which is perceived only by instruments and calculation; and of these motions, some are grasped after a very long period of time, and others after a short one. There is also a third motion, which is demonstrated by reason; for the motion of the wandering stars is found at one time to be more rapid and at another slower; and sometimes a planet seems to be moving forward, and sometimes backward. And because this cannot be in keeping with the nature of a celestial body, whose motion ought to be regular in all respects, it has been necessary to posit different motions by which this irregularity might be reduced to a fitting order.

             2566. As to the number of planetary motions, let us now state what the mathematicians say about this, so that with this in mind we may conceive some definite number. But as to the other things which have not been stated, we must either investigate these for ourselves or in this matter accept the opinion of those who do investigate the problem. The same thing applies if some view should appear later on in addition to those which are now stated by men who treat this kind of problem. And since in choosing or rejecting opinions of this kind a person should not be influenced either by a liking or dislike for the one introducing the opinion, but rather by the certainty of truth, he therefore says that we must respect both parties, namely, those whose opinion we follow, and those whose opinion we reject. For both have diligently sought the truth and have aided us in this matter. Yet we must "be persuaded by the more certain," i.e., we must follow the opinion of those who have attained the truth with greater certitude.