It's not the truth/Ain't actual/Everything is counterfactual
Having linked to this article of the Summa Theologia in a recent post, I was struck by how -- well, Geoff used "pretty mealy-mouthed," but I'll go with "tempered in his support" St. Thomas is regarding the opinion that the Son would not have become man had man never fallen. He writes:
There are different opinions about this question. For some say that even if man had not sinned, the Son of God would have become incarnate. Others assert the contrary, and seemingly our assent ought rather to be given to this opinion.
Here, "seemingly" translates the Latin "videtur," which is the first word of the first objection in every article in the Summa. "It would seem that...," St. Thomas writes again and again, only to go on to argue that what seemingly is, is not.
Seemingly, his opinion on whether, if man had not sinned, God would have become incarnate was more doubtful than his opinion on -- to choose the next article -- whether God became incarnate in order to take away actual sin, rather than to take away original sin:
It is certain that [certum est] Christ came into this world not only to take away that sin which is handed on originally to posterity, but also in order to take away all sins subsequently added to it....
To try to figure out why St. Thomas hedges his bet on the question of the Incarantion, look at the reason he gives for why "seemingly our assent ought rather to be given to" the opinion that the Son would not have become incarnate:
For such things as spring from God's will, and beyond the creature's due, can be made known to us only through being revealed in the Sacred Scripture, in which the Divine Will is made known to us. Hence, since everywhere in the Sacred Scripture the sin of the first man is assigned as the reason of Incarnation, it is more in accordance with this to say that the work of Incarnation was ordained by God as a remedy for sin; so that, had sin not existed, Incarnation would not have been.
Notice how modest St. Thomas's concept of theology is. As he says way back in the beginning of the Summa, "sacred science [sacra doctrina] is established on principles revealed by God." God revealed that His Son became man because of Adam's sin; God did not reveal that His Son would have become man regardless; therefore "it is more in accordance with" God's revelation that His Son would not have become man if man hadn't sinned.
You could say that St. Thomas advocates using a "minimum speculation" measure in choosing between uncertain opinions. Might we even consider it a version of Ockham's Razor?
Now, there are two things to keep in mind in applying such principles of parsimony. One is that the proper measure -- of "speculation," as I called it here, or of "plurality," as in Ockham's formula -- is not always evident. I have to think one reason St. Thomas saw the No Incarnation Without Original Sin opinion as "more in accordance" with Scripture is that it was the opinion of St. Augustine, and St. Thomas did not lightly disagree with him. Others may find St. Augustine less compelling on this point.
The other thing to keep in mind is that God is not bound by parsimony, which is why Ockham's Razor is properly understood as a guide for constructing theories, not proving facts. It's also why St. Thomas is careful to say "seemingly" and "more in accordance with," not simply "is."
And if he doesn't think it can be known for certain what God would have done, there's no question in his mind about what He could have done:
And yet the power of God is not limited to this; even had sin not existed, God could have become incarnate.
And if you think this whole question of a counterfactual Incarnation is utterly irrelevant, Irish Catholic and Dangerous sketches some of the theological implications of the different opinions.