What that means is that this is a statement of the essentials of the dogma. If somebody wants to talk about the dogma, he's got to take all the pieces into account or he's not talking about the dogma. It's a singular privilege and grace; it's done by the Omnipotent God; it's done in virtue of the merits of Jesus Christ (so, in other words, Mary didn't do it herself and it depends on what Jesus did), and she was sinless from the very beginning of her existence.
It's in no way an argument. It's, so to speak, the rules of what the dogma is. You'd have to read elsewhere in the encyclical to find where the arguing part happens.
And I thought I said that it was only in light of the dogma that I would say it was "fitting." God does everything well. If you think God 'did' the Incarnation and you think He 'did' the Immaculate Conception, then you can enjoy the thought that the perfect man was conceived and gestated and all the rest by a sinless mother.
But no serious theologian argues that because anything else would have been "unfitting" therefore He MUST have done the IC.
Again, I hope that's clear. It is as if you were looking at a law that was passed and expecting it to contain the explanation for why it was passed, or reading the verdict and expecting it to contain the evidence. I hope those analogies are useful.
This is a good point and leads me to make another:
As often happens, either naturally or by intent, we've drifted away from the original discussion. What was posted was a logically syllogism and the debate was over its validity, truth, proof. The rules of formal logic are quite different from the rules of a court and very different from the rules of doctrine.
If we stay on topic, we are under the rules of formal logic, and here, only valid syllogism - using deductive reasoning - are considered "proven" or true.
Inference can be evidence, but it can't be "proof" under the rules of formal logic. MD's "fitting" comments are perfectly valid inferences, can be considered evidence, but are not deductive proof, and as I read it they weren't intended to be.
TC's IC premise was claimed as a hidden necessary premise and used initially as a means to build a syllogism that disproved MD's conclusion "Mary is the mother of God."
My response was A) This premise can be true or false and not effect the conclusion and is therefore irrelevant; and B) The Church does not maintain that the IC was logically necessary (or necessary at all) for the Incarnation - and therefore irrelevant again.
Unless, TC comes up with a different approach or with another way to tie the logical necessity of the IC to the Incarnation, then, thus far, the search for a "simple syllogism to prove Mary is the mother of God is false" remains elusive.