Twins aren't an argument against conception any more than clones are. Both twins lives began at conception, and they separated later.
That's not how twins work though. The difference between an embryo that splits into identical twins and one that doesn't is probably nothing, just random chance that splits them.
The same if you make clones from a cell line by splitting off cells, then implant them. Unless you believe in determinism? So unless you do, they both
don't begin existing at conception.
The problems you list are entirely pedantic and without substance, though. By your own standards, the notion of brain activity is clearly nonsensical because there's no sharp line where brain activity begins, as soon as the neural plate forms in the first few days, the first signals begin and they steadily strengthen as the plate forms the neural tube and then grows into a full brain. People may say "Brain activity at five weeks" but this is shorthand for the actual situation where it steadily develops, a line where they feel there's enough brain activity and it's detectable to current instruments. You're just drawing an arbitrary line and saying "This much brain activity, this many neurons."
The line is thought. That
is a precise line, but where it exists is somewhat scientifically blurry. Brain activity is a prerequisite though, and that starts at about 8 weeks, so I'd draw the legal bar there to be safe. Neuron's firing in the spine but not being picked up isn't something I'd call human thought, any more that pain that is never felt by a brain because of a broken spine.
Wow, so you split the definition in half in your reply so that you could wax pedantic about each half separately and each of your disputations is neatly canceled by the other half you split off. Pretty decent proof you can't actually argue with it else you wouldn't have needed to use such a dishonest method.
Miscommunication. Here is your definition:
As far as defining human we already have a fairly thorough scientific definition, that generally boils down to the Homo Sapiens Sapiens genome though there's some discussion of whether Neanderthals and Denisovans should be included. As there are no extant examples those have no impact on the matter at hand. This definition includes various specific traits such as upright bipedal posture, member of the great ape grouping without a tail or prehensile toes, etc. Nobody worries about edge cases like a guy who lost a leg to a landmine making "biped" no longer correct for that definition.
I read your definition as the bold part, and then you as claiming it implies the unbolded part. I then pointed out that the bold part doesn't imply the rest, which is one of the advantages of what I thought your definition was. Then I also add that what I thought your definition was isn't specific enough.
If you actually intended both parts as your definition, your definition is just bad
because it doesn't include the edge cases. A definition for something so fundamental that doesn't include edge cases is fatally flawed. Like there are thousands counterexamples in every city bad.
Basically, I was trying to be charitable to you.
Look, again, this is the philosophy forum. One of the most famous counterexamples in philosophy was Diogenes Chicken. If your definition falls to something so obvious ("Behold, an amputee is not a man"), it needs serious work.
Pedantry that's irrelevant to the question, is the problem. Again, we haven't had to redefine human away from biped legally just because a dude lost a leg to a landmine.
Um, yeah, you do. You 100% do. That's the basic point of a philosophical definition. We aren't looking for something for colloquial or good enough use, we are looking for something the law might use after being bent by a lawyer.
There are also deeply disturbing edge cases based on brain function to address since that seems to be the mode of this discussion. Who gets to define what brain function is? Suppose someone clones only a small segment of neurons but they start sending bioelectrical signals in a petri dish? Is the petri dish a human now? How far away from the human norm does it have to be before something is no longer human and you take away their rights? A person with Alzheimer's has brain activity but the personality is gone, are they still human? A person with ADHD or Autism has abnormal brain function unlike other humans, if they count as human how do you justify not defining other hominids with nonhuman brain function? Do you trust that future politicians aren't going to make it stop counting once you've made "brain function" such a critical lynchpin?
See, the petri dish! Now that's a good counter example! (The rest I'll deal with quickly by noting I'm including any thoughts, so it doesn't matter if they are normal, human, or whatever. The human classification
was the genome test)
This shows that my previous definition was too broad or alternatively not precise enough by not defining brain. Or maybe this should be a human? (You had something of the same objection above with the neural plate, but I feel that we can better know scientifically when thoughts start because we know something about how humans develop, whereas this would be new).
Now that is a question, at what point do a bunch of neurons lumped together make thoughts? And should it be considered human if they are all human neuron cells? I really don't know. Though I will note that conception has the same problem: what if you do this from a stem cell line? Is that a human? Does it matter if the stem cells came from a fertilized egg or skin cells (real question about this one)?