“The products of conception are alive at all times - until they die - and they are human at all times. There is no point where the human ovum isn't, well, human.”
Well yes, but you miss my point. Any part of a human, removed or ejected from a human, dead or alive, is of human origin. A gamete, a cell, or a tissue, or a product of conception, a collection of tissues and organs, can all be human and all be alive.
My point is, not all these things are human beings. Clearly ejaculated semen is human, but not a human being. I would argue that a live newborn baby is a human being.
“I don't think biology is any help to you here. Your question is fundamentally philosophical, not scientific.”
Philosophically we can define what a human being is. Once we have such a definition we can test this definition scientifically. We can biologically determine where, along the line from sperm and ova, through zygote, embryo, foetus, newborn, toddler, to teenager, where according to our philosophical definition, a being a human being starts in the reproductive cycle.
I would also argue that we can use science and biology to inform our philosophy and ethics. Philosophy and ethics do not work in a vacuum, they are influenced by the facts and fictions of the world, and science strives to bring facts to light.
I take your point. It’s important, though, to recognise where science can help us, and where it cannot.
You’re using the term “human being”. You don’t define the term, beyond saying that a full-term baby is a “human being”, but from the way in which you use it I think it’s fair to say that you use it to mean something like “a human individual which is developed to a point at which it is not ethical to abort it”.
That’s obviously a philosophical or moral concept, not a scientific one. Not only does the term have a fundamentally moral meaning – it tells us whether a particular act is ethical or not - but it also presupposes that the degree of respect to which a human individual is entitled depends on the state of its development, which itself is of course a moral claim.
Science can certainly illuminate this discussion by telling us how developed a particular human individual is, at least to some extent. If we claim that, say, an individual which can feel pain is a “human being” in this sense, then science can help to tell us at what stage of development it can feel pain, or at least it can identify a stage of development at which it cannot feel pain. But what science cannot do is to validate (or refute) the claim that an individual which feels pain is therefore a “human being” in the sense described.
From the moment of conception, science tells us that the fertilised ovum is a genetically unique living human individual, distinct from both its father and its mother. Science also tell us that at that moment it lacks any distinct organs, and that only at later stages - if it lives so long - will it acquire nerves, any cerebral function, the capacity to react to stimulus, instinct, volition, self-awareness, the capacity to breathe, the capacity to digest, vision, hearing, speech, the capacity to reason, to fall in love, to dream, to compose an opera, to watch reality television, etc.
But science has nothing to say about the moral significance of being a human individual, or of any subsequent developmental marker. Picking a particular capacity and saying that an entity needs to have that capacity in order to be a “human being”, in the sense in which I think you are use in the word, is a moral claim, not susceptible to any kind of scientific testing, validation or support.