Now, what about the sub-conclusion in lines 8-9 of the argument? It infers that 'one must posit something the existing of which is necessary'. A review of G6 up to this point shows that at least part of what can legitimately be meant here by saying of something that its existing is necessary is that it exists, but it couldn't have been generated and it can't be destroyed. We've already seen that 'altogether immutable' Alpha could never have begun to exist and can never cease to exist. Could Alpha on those grounds be identified as this necessarily existent thing that 'one must posit'? No—or, at any rate, not yet.
Aquinas understands generation and destruction as including all the natural processes of being brought into and taken out of existence. And in his Aristotelian view of nature, some actually existent things—the sun, for instance—exist necessarily in the special, narrow sense of not being subject to any natural processes of beginning and ceasing to exist, and yet they exist dependently. Unlike all the other things '[w]e see ... in the world', a dependently necessary being is independent of all natural originating and sustaining causes. But the sun's nature doesn't entail its existence any more than the nature of the carrot I'm about to eat entails its existence. The (Aristotelian) sun's existing independently of natural generation and destruction warrants its being described as necessary in this special sense, while the fact that its nature does not entail its end p.109
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© Copyright Oxford University Press, 2006. All Rights Reserved existence dictates its being described as having 'the cause of its necessity in something else' (lines 10-11)—something else that sustains it in an
existence that is not subject to the vicissitudes of nature.
31 Human souls and angels are among the things that Aquinas thinks exist necessarily in this sense, because he takes them to involve no matter, and therefore to be invulnerable to natural disintegration. But since their natures do not entail their existence, they, too, exist dependently. For a helpful critical survey of Aquinas's views on this topic, see Brown 1964.
Although at least many, maybe all, of Aquinas's dependently necessary beings are scientifically discountable, his introduction of them in lines 9-11 of G6 is justified dialectically. The argument aims at showing that 'one must posit some first necessary being that is necessary through itself'. But since Aquinas and his contemporaries believed in 'necessary beings that have the cause of their necessity in something else' (lines 12-13), he has first to rule out those lesser necessary beings. He does so by denying the possibility of an infinite regress of dependently necessary beings (in lines 11-13) along the lines of the analogous denial in lines 6-7—the one we've already looked at. If we then move directly to G6's conclusion that 'one must posit some first necessary being that is necessary through itself', we have arrived justifiably at an entity that can and must be identified with altogether immutable, beginningless, and endless Alpha.
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