We now continue looking at the Modal Third Way. We have already established that a temporally necessary being exists. Our aim now is to find a bridge between this and God. What reason do we have for concluding that this temporally necessary being is God?
7. Whatever is temporally necessary might be unlimited.
If anything is unlimited, it is most plausibly a temporally necessary being. After all, a thing which is temporally contingent is limited in its duration, so we can, at any rate, rule out the great majority of things as being unlimited. There is no contradiction in the idea of something being both a) temporally necessary; and b) unlimited - at least, there's no contradiction I'm aware of. This entails that there is at least one possible world in which we have a link from temporal necessity to unlimited essence.
We may now, as Maydole muses, "work on establishing a linkage between unlimitedness and explicability."
8. Whatever might explain itself is unlimited.
We may legitimately envisage a possible world in which the temporally necessary being is a first cause of the existence of all other things. From this, we may infer that since such a being cannot be caused by anything else (or else it wouldn't be "first" to begin with), this being must of necessity not be limited by any external object.*
9. Nothing which is unlimited can be explained by anything else.
This premise, I think, is very plausible. As we saw above, a thing X is limited by Y if Y explains X. For example, the sun's rays are explained by the sun itself, and so we conclude that the power of its rays is limited by the power that is produced by the sun.
10. Everything which is unlimited is supreme.
The temporally necessary unlimited first cause must be supreme. For, that is part of what we mean by "supremacy." Moreover, there can be only one Supreme Being. Imagine that both A and B are supreme. If A is distinct from B, then presumably A lacks something that B possesses, or vice-versa. However, a thing can only lack something if it is limited, so whatever is unlimited (and by extension, supreme) must of necessity be one.
11. Therefore, there exists a Supreme Being.
The argument for a temporally necessary being is, I believe, airtight. We are doing some metaphysics as we make a link from this to the conclusion that God (or, a Supreme Being) exists, but there is nothing wrong with that. I do think we could argue inductively in order to close the gap, however.
Consider the fact that our observations lead us to believe that whatever is limited is temporally contingent. The hardness of a rock, for instance, is limited. We also find that its durability is limited. On the basis of this example, among many others, we generally draw an inference from limitedness to temporal contingency. If, however, a thing is temporally necessary, then its duration is unlimited, since it exists at all times and cannot cease to be, at least in any of the worlds in which it does exist. This ought to give us an incentive to believe that whatever is temporally necessary is also unlimited in other respects, e.g. power. But since a thing is unlimited only if it is supreme (tautology), it follows that a Supreme Being exists in whatever world there is something temporally necessary. And, because a temporally necessary being exists in the actual world, it follows logically and inescapably that a Supreme Being exists in the actual world.
One shortcoming of the MTW, as Maydole himself acknowledges, is that it does not establish that a Supreme Being exists in all possible worlds. For, there are possible worlds in which nothing presently exists, in which case nothing is explicable and there is no need to appeal to a temporally necessary being.
Another drawback is that the MTW establishes only the existence of a Supreme Being; it does not demonstrate the truth of theism per se. At least on the surface of it, there are versions of pantheism that are just as consistent with the conclusion of the MTW as theism is. This shouldn't be disheartening for the theist, though. After all, the MTW does establish a conclusion that flies in the face of (capital "N") Naturalism, and establishing the rationality of an alternative worldview to Naturalism is one of the goals of natural theology.
For an original paper on the Modal Third Way, see: Aquinas' Third Way Modalized
*I'm using the terms, "object," "thing," and "being" interchangeably.