I've given a lot of thought as to the semantics of this, and this is what I've come up with: everything has a definitive beginning.
Let's talk about stars, for now. A cloud of gas exists in space, say. This cloud begins to swirl. Is it a star? What is a star? A star is gas that is so hot from gravitational compression that fusion begins and it "burns" (also, to note, it must be seperate from other such gasses; a star is not made up of billions of stars) So this gas is not a star. When it gets to the point that the first atom fuses, then I would call that the beginnings of a star, and, since it meets the criteria, a star (fusing gasses).
So the star has a beginning in which it didn't quite look like a star as we know it, but performed in exactly the same way. So a person's life could be measured in the same way. There exists a point zero in which life begins. Before that, life is not possible, after this life exists and will, if unobstricted/impeded, keep existing.
Is it breathing? Is that the definitive moment? Hold your breath. Are you dead? Then that's not it.
Heatbeat? The French helped us with this. If your head is removed from your body, you are conscious for at least fifteen seconds, without blood passing through. So while it's important, it's not the "spark of life" as we can do without it for a time, like breathing.
Birth? Does life begin when you exit your mother? That many people remember, somehow, exiting the womb, or even before, would seem to indicate that this is not significant. While a milestone event, the event itself is not the giver of life.
That's what it comes down to, right? What event is the giver of life? While other things happen to and around the beginnings of a person, what is the moment that it is really alive? Let's keep going.
Is it the second trimester? At this point a person resembles a person and the heart starts for itself. We've ruled out the heart argument, so let's look at the person. Does having limbs, useful or not, make you human? Ask war survivors without any limbs to answer that. So it's not appearance. Further back...
Is it having eight cells? Is there something significant about attaining this state that makes a person a person? If I never reached this state, would I be me? Obviously I would not have been born, but does this prevent the genetic material from becoming what it will finally be? Not really. It's a progression on a chart, a stage. Just like the beating of a heart, birth, and the first breath, it's an event, a stage, but not a defining moment.
Before conception, are you who you are? You're not. You're partially this and possibly that. You know what half of you will be (ignoring dominance) but not the other half. At this moment you could be almost anything (within human limits). Something happens here that defines you and who you will be. Something is given here that starts a chain reaction that, if allowed to continue, will result in a person, you. Before this, there is no hope of you appearing as you did. After this, there are no changes that can take place that would make you. This is where that happened. This is your defining moment where you became you.
No, it doesn't look like a person. It doesn't have all the organs, or nerves of a human. But just as a protostar is a form of star, so a protohuman is a form of human. The egg will now start to divide (a sign of cellular life, at least) and slowly form a person. Does that make it alive, then? Absolutely. Petri dish or uterus, the joining of the two cells begins life, and, thus, is alive.
[Std. Disclaimer: My opinion, and an answer to a question about said opinion.]
The cynical can often see the sinister aspect of a cup of coffee if given enough time.
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