22. The short form of the Third Way is as follows:
A. In nature we observe that things are generated and corrupted, i.e. that they can be and not be.
B. But that which is generated at some point is not. Whatever can be and not be, does not always exist.
C. Therefore if everything is of this sort, then everything must have come to be at some point.
D. [Implicit: But if it came to be as the addition of a form to some preexisting matter, then either the matter always was, which would violate our premise, or it came to be out of some other matter, etc.]
E. Therefore if Everything is merely possible, i.e. is capable of being and not being, and therefore is generated out of something else, then there must have been a time when nothing (neither present things, nor the matter of which they are made, nor any matter at all) existed.
F. But if there were a time when nothing existed, then nothing would presently exist.
G. This is absurd, since clearly things exist presently.
H. Therefore there must be something which is not merely possible, but necessary, i.e. something out of which other things are generated but which is itself ungenerated.
I. But this thing or set of things either has its existence simply on its own account, or on account of another necessary principle.
J. But the regress of necessities cannot go on forever.
K. Therefore there must be a first necessary being, which is the ground of its own existence and necessity.
L. This is what we call "God."
23. There are two significant difficulties in reading the Third Way. The first is that modern readers, acquainted with the arguments of post-Enlightenment rational theology, expect the Third Way to be a version of the cosmological argument (I.e., a reduction of the principle of sufficient reason to a reason which is its own necessity.). This expectation, when it is not met in the text, tends to make Thomas's argumentation seem bizarre and opaque.
24. The second difficulty is in recognizing the importance of Thomas's initial reference to generation and corruption in properly framing the argument. Here I owe a debt to James Chastek for giving a sound interpretation of the text. Thomas's conception of possibility revolves around the fact that things are and are not at different points in time: in other words, that they come to be out of certain things, and decompose back into the matter of which they were made. Chastek's insight is to see that the line "if everything were possible not to be, then at some time there could have been nothing in existence" is not best parsed as a statement of modal logic, but as a gesture toward a universe without any underlying principle of material conservation.
25. Thomas's thought is not that, because everything presently existing must at some point not have existed in the past, there must have been a time when nothing existed. That would be a faulty inference. Rather, he is saying that if everything were simply contingent, such that everything were generated at some point, including the matter out of which everything is made, down to the most fundamental and basic stuff of the universe, then there would have been a time when nothing existed at all. From this point the argument is easy to follow.