Wow, that was fast thinking.
If I had been there at the spot, I could not have said anything.
But now that I've had a chance to think for ten minutes, here's what I come up with:
You and Mark were debating about whether the unborn is alive, in the first place, because of a shared assumption, or let's say at least a shared overlap between your assumptions and his, that the unborn's aliveness is morally relevant to the question of abortion. Not necessarily a sufficient determinant in that question, but relevant.
Let's say the two of you shared the premise "The aliveness of the unborn, if it is a fact, carries x weight in favor of the unborn, in relation to the question of abortion."
You convinced Mark that the unborn is alive. But you did that by convincing him that your definition of "alive" was correct. In other words, you convinced Mark that the unborn is alive, but alive according to your definition.
Now, I would guess that before you and Mark started debating, "The unborn should not easily be aborted . . ." was a pre-logical moral intuition on your part, and ". . . for one thing, because its aliveness (according to biologists' definition) carries x weight" was an attempt, perhaps a reasonable one, to logically explain your intuition to an extent.
(Compare how, in “De Facto Guardian and Abortion,” Steve Wagner, Timothy Brahm, et. al. find their pre-logical moral intuitions that Mary should be legally obligated to feed the child; then they proceed, in a section called “Making Sense of Our Intuitions,” to cogitate logically about the morally-relevant factors -- and to develop some taxonomy, “de facto guardian.”)
Or if "The unborn should not easily be aborted . . ." was not a pre-logical moral intuition on your part, but arrived at through logic, then I would guess that to reach that logical conclusion you would have had to resort to some other pre-logical moral intuition, so that ultimately the basis of your moral principle was pre-logical. (If I'm being too presumptuous about this, please correct me.)
And I would guess that before you started debating, "The unborn can easily be aborted . . ." was a pre-logical moral intuition on Mark's part, and ". . . for one thing, because it is not alive (according to the NASA definition)" was an attempt, perhaps a reasonable one, to logically explain his intuition.
I'm not sure that -- even if Mark was correct that his moral intuition "The unborn can easily be aborted . . ." stemmed in part from the non-aliveness of the unborn -- his having to change his view about the aliveness would have changed his moral intuition about abortion. Because that moral intuition stemmed from non-aliveness according to one definition, and the same intuition might still stem from aliveness according to another definition.
Specifically, Mark might now say, as some pro-choicers do, "Even though an organism is living, it has no right to continue to live unless it is self-sustaining."
I'm not saying that that would be duplicity or rationalization on Mark's part. If dependence really did, intuitively, seem morally relevant to him, he would have remained consistent; he would simply be expressing his consistent view using some new learning that he has acquired in the semantic dimension.
The real difference between you -- and me -- on the one hand, and Mark on the other, might be just different intuitions about the moral relevance of dependency at a certain stage of development.
My points in this comment have been, first, that the definition of "alive" is ultimately only of semantic importance. (Though losing the ability to bandy about the term "not alive" must have ramified into causing Mark to examine his views more carefully.)
And secondly, that our moral principles are ultimately based on something pre-logical, and can neither be fully arrived at through logic, nor fully proved correct through logic. "Logic may lead us close to the door of intuition, with luck very close, but in order to pass through and feel the intuition, feel that something matters, we have to leave logic behind."
The quote is from a blog post in which I have thought as best I could about these things: