Objection 1. It would seem that the lights of heaven are living beings. For the nobler a body is, the more nobly it should be adorned. But a body less noble than the heaven, is adorned with living beings, with fish, birds, and the beasts of the field. Therefore the lights of heaven, as pertaining to its adornment, should be living beings also.
Objection 2. Further, the nobler a body is, the nobler must be its form. But the sun, moon, and stars are nobler bodies than plants or animals, and must therefore have nobler forms. Now the noblest of all forms is the soul, as being the first principle of life. Hence Augustine (De Vera Relig. xxix) says: "Every living substance stands higher in the order of nature than one that has not life." The lights of heaven, therefore, are living beings.
Objection 3. Further, a cause is nobler than its effect. But the sun, moon, and stars are a cause of life, as is especially evidenced in the case of animals generated from putrefaction, which receive life from the power of the sun and stars. Much more, therefore, have the heavenly bodies a living soul.
Objection 4. Further, the movement of the heaven and the heavenly bodies are natural (De Coel. i, text. 7,8): and natural movement is from an intrinsic principle. Now the principle of movement in the heavenly bodies is a substance capable of apprehension, and is moved as the desirer is moved by the object desired (Metaph. xii, text. 36). Therefore, seemingly, the apprehending principle is intrinsic to the heavenly bodies: and consequently they are living beings.
Objection 5. Further, the first of movables is the heaven. Now, of all things that are endowed with movement the first moves itself, as is proved in Phys. viii, text. 34, because, what is such of itself precedes that which is by another. But only beings that are living move themselves, as is shown in the same book (text. 27). Therefore the heavenly bodies are living beings.
On the contrary, Damascene says (De Fide Orth. ii), "Let no one esteem the heavens or the heavenly bodies to be living things, for they have neither life nor sense."
I answer that, Philosophers have differed on this question. Anaxagoras, for instance, as Augustine mentions (De Civ. Dei xviii, 41), "was condemned by the Athenians for teaching that the sun was a fiery mass of stone, and neither a god nor even a living being." On the other hand, the Platonists held that the heavenly bodies have life. Nor was there less diversity of opinion among the Doctors of the Church. It was the belief of Origen (Peri Archon i) and Jerome that these bodies were alive, and the latter seems to explain in that sense the words (Ecclesiastes 1:6), "The spirit goeth forward, surveying all places round about." But Basil (Hom. iii, vi in Hexaem.) and Damascene (De Fide Orth. ii) maintain that the heavenly bodies are inanimate. Augustine leaves the matter in doubt, without committing himself to either theory, though he goes so far as to say that if the heavenly bodies are really living beings, their souls must be akin to the angelic nature (Gen. ad lit. ii, 18; Enchiridion lviii).
In examining the truth of this question, where such diversity of opinion exists, we shall do well to bear in mind that the union of soul and body exists for the sake of the soul and not of the body; for the form does not exist for the matter, but the matter for the form. Now the nature and power of the soul are apprehended through its operation, which is to a certain extent its end. Yet for some of these operations, as sensation and nutrition, our body is a necessary instrument. Hence it is clear that the sensitive and nutritive souls must be united to a body in order to exercise their functions. There are, however, operations of the soul, which are not exercised through the medium of the body, though the body ministers, as it were, to their production. The intellect, for example, makes use of the phantasms derived from the bodily senses, and thus far is dependent on the body, although capable of existing apart from it. It is not, however, possible that the functions of nutrition, growth, and generation, through which the nutritive soul operates, can be exercised by the heavenly bodies, for such operations are incompatible with a body naturally incorruptible. Equally impossible is it that the functions of the sensitive soul can appertain to the heavenly body, since all the senses depend on the sense of touch, which perceives elemental qualities, and all the organs of the senses require a certain proportion in the admixture of elements, whereas the nature of the heavenly bodies is not elemental. It follows, then, that of the operations of the soul the only ones left to be attributed to the heavenly bodies are those of understanding and moving; for appetite follows both sensitive and intellectual perception, and is in proportion thereto. But the operations of the intellect, which does not act through the body, do not need a body as their instrument, except to supply phantasms through the senses. Moreover, the operations of the sensitive soul, as we have seen, cannot be attributed to the heavenly bodies. Accordingly, the union of a soul to a heavenly body cannot be for the purpose of the operations of the intellect. It remains, then, only to consider whether the movement of the heavenly bodies demands a soul as the motive power, not that the soul, in order to move the heavenly body, need be united to the latter as its form; but by contact of power, as a mover is united to that which he moves. Wherefore Aristotle (Phys. viii, text. 42,43), after showing that the first mover is made up of two parts, the moving and the moved, goes on to show the nature of the union between these two parts. This, he says, is effected by contact which is mutual if both are bodies; on the part of one only, if one is a body and the other not. The Platonists explain the union of soul and body in the same way, as a contact of a moving power with the object moved, and since Plato holds the heavenly bodies to be living beings, this means nothing else but that substances of spiritual nature are united to them, and act as their moving power. A proof that the heavenly bodies are moved by the direct influence and contact of some spiritual substance, and not, like bodies of specific gravity, by nature, lies in the fact that whereas nature moves to one fixed end which having attained, it rests; this does not appear in the movement of heavenly bodies. Hence it follows that they are moved by some intellectual substances. Augustine appears to be of the same opinion when he expresses his belief that all corporeal things are ruled by God through the spirit of life (De Trin. iii, 4).
From what has been said, then, it is clear that the heavenly bodies are not living beings in the same sense as plants and animals, and that if they are called so, it can only be equivocally. It will also be seen that the difference of opinion between those who affirm, and those who deny, that these bodies have life, is not a difference of things but of words.
Reply to Objection 1. Certain things belong to the adornment of the universe by reason of their proper movement; and in this way the heavenly luminaries agree with others that conduce to that adornment, for they are moved by a living substance.
Reply to Objection 2. One being may be nobler than another absolutely, but not in a particular respect. While, then, it is not conceded that the souls of heavenly bodies are nobler than the souls of animals absolutely it must be conceded that they are superior to them with regard to their respective forms, since their form perfects their matter entirely, which is not in potentiality to other forms; whereas a soul does not do this. Also as regards movement the power that moves the heavenly bodies is of a nobler kind.
Reply to Objection 3. Since the heavenly body is a mover moved, it is of the nature of an instrument, which acts in virtue of the agent: and therefore since this agent is a living substance the heavenly body can impart life in virtue of that agent.
Reply to Objection 4. The movements of the heavenly bodies are natural, not on account of their active principle, but on account of their passive principle; that is to say, from a certain natural aptitude for being moved by an intelligent power.
Reply to Objection 5. The heaven is said to move itself in as far as it is compounded of mover and moved; not by the union of the mover, as the form, with the moved, as the matter, but by contact with the motive power, as we have said. So far, then, the principle that moves it may be called intrinsic, and consequently its movement natural with respect to that active principle; just as we say that voluntary movement is natural to the animal as animal (Phys. viii, text. 27).
Source: Summa Theologica (New Advent)