If X is a part of a human being, then X is not a human being. No part can be a whole of the same sort as the part. That's ontology 101.
- A8
True, a part is not the same as the whole. But, what are you referring to specifically?
Yes, and? As I said, the question is when the life form or forms stops being a part of the set of cells or individuals that will eventually be known as "biological mother" and "biological father" and start being an individual that will be known as "biological child". The obvious tautology here is that an individual starts when they cease being a part of another individual (see also twinning as well as fertilizatoin) and can also stop being an individual if they become part of another individual (see chimeras).
Let's say I use the term Y to mean "some wood."
Then wouldn't every Y either be one molecule in size or contain multiple other Ys?
And let's say Z means "some human tissue"... wouldn't every Z either be one molecule in size or contain multiple other Zs?
So that rule (by tautology) applies only to entities and collections incapable of containing like entities/collections
For instance, if A refers to "a collection of tools," an A can contain other As. And if B refers to "a single hammer," then a B cannot contain other Bs. And if C refers to "a collection of tools that is exactly one hammer, one saw, and one screwdriver," then a C cannot contain other Cs, even though it is a collection. I mention this only to point out that collections can meet the definition you give above; collections are important later.
Of course, living human tissue is not the same as a human life, even though living human tissue is alive and exhibits behavior directed by human Chromosomes. A purely mechanical definition of "a human life" would be rejected by most people. Obviously, "a human life" is not "a collection of human body parts that work together to stay alive," because there are amputees (missing parts of the collection), people with artificial organs, and collections of organs that can survive together (with the help of machinery) but are not what we would consider "a human life" (for instance, if you rigged a stomach and intestines together in nutrient fluid, they could probably survive, especially if you send the right nerve signals electrically)
So what is a "human life?" Is it a single, discrete element? Or is it a collection of elements? I suggest that a "human life" is the same as a "human mind"--wouldn't a human mind removed from its human body still be a "human?" Wouldn't the "lifeless" body no longer be connected to a human life? It would certainly not consist of one.
Even Christians generally consider the soul to be tied to the mind. For instance, the soul is considered responsible for crimes committed by the mind or at its direction. When a Christian claims that during surgery his/her soul was drifting away "towards the light," but then came back into the body, their minds were able to observe the activity of and/or sensory input from their souls, suggesting a connection between the two. Most Christians probably believe their minds will be with them in Heaven, and are inseparable from their souls... others might see Heaven as a general closeness to God that is uncognizable, and believe their minds will die and be left behind on Earth... and still others believe other things. But for the main group, a human life "ends" when the mind leaves the body to travel with the soul to Heaven.
So we agree that the mind either dies or leaves, and that is when there is "death" (the end of a human life).
So now that we have established the importance of a human mind, what the heck is it? From WikiPedia:
Other philosophers contend that some nouns are not names of entities but are a kind of shorthand way of referring to a collection (of either objects or events). In this latter view, mind, instead of referring to an entity, instead refers to a collection of mental events experienced by a personIf a "human mind" is a collection of mental elements, what happens if some of those elements are missing? For instance, some people suffer brain damage and miss the use of some structures up there (losing short-term memory, for instance, or having their consciences ripped out, as was the case with Phineas Gage). Or if they don't lose completely the use of such structures, they lose partial capability... further supporting the idea that human life is in fact not something that can be pronounced "on" or "off."
And if a "human life" is just "a brain that can keep a body alive," then a brain stem could be part of what qualifies as a human life. Even reptiles have brain stems.
As for your contention that a human life cannot be a part of another human life, I would have to disagree. If in the future technology enables us to combine minds or mix-and-match brain structures between people, our conception of "human lives" as distinct from one another would crumble.
And once we allow that "human lives" might not be discrete, the notion of ensoulment crumbles. After all, ensoulment requires persons to be responsible for their actions (which then reflect on the quality of the soul). But how would you judge the crimes committed by a "person" whose brain was made up of parts from three distinct people? Which soul(s) would go to hell for crimes committed by it?
Even if you could point to a single brain structure to which the soul was bound, that wouldn't help. What if scientists could cut that structure in half and splice in half of someone else's? Whose soul would be where? It's foolish to talk of souls.