By "beings in which there is no matter," St Thomas meant pure spirits. By "beings in which the matter is not open to receive another form," he meant the heavenly bodies: if he had written in our time, he might be well taken to mean those primitive atoms or molecules, which have been termed "the building stones of the universe." He has in his eye the whole class of natural objects, animate and inanimate, that can neither destroy themselves nor ever be destroyed and broken up by any of the ordinary processes of nature, but are permanent from age to age, whether existing apart or in composition. In the physical order, of which St Thomas here speaks, the existence of these beings is "absolutely necessary"; no physical force can destroy them. One might say the same of the total store of energy in the universe, according to the principle of the 'conservation of energy.' St Thomas's acquaintance with Plato was through the Neo-Platonists; and their favourite Dialogue was the Timaeus, the following passage of which (Tim. 41) well illustrates his meaning. The Platonic Demiurge is addressing the minor deities whom he has compounded, them and their offspring: "Ye gods, god born, works of my fatherhood and constructive power, what has been made by me is indissoluble, so long as it has my consent to its being. Whatever is bound and put together may indeed be loosened: but it were ill done to undo a work fairly compacted and well made. Therefore, made as ye are, ye are not absolutely beyond death and dissolution: still ye shall never be dissolved nor meet the doom of death, finding in my will a tie greater even and more potent than the ties wherewith your being was originally bound together."
Of God and His Creatures: 2.30