I am pretty comfortable calling a body without a head or brain "not human" (this was what the OP suggested).

> ... the unborn are certainly "human".

You would be surprised how many people would disagree with you about that. I personally definitely would not call a freshly-fertilized egg in a womb a human. I would agree that the fetus just before birth is human. I do not have an answer for how far back you have to rewind for the unborn fetus to not be human (no one has a universal answer to this).

More radically, you can also go in the opposite direction. Infants have no memories for the first year or two. They see upside-down at first. Brain development is clearly continuing after birth. So you might be able to justify infanticide for some time after birth. I don't recommend this, of course.

Right, Peter Singer is good on this topic.

There's no sense in assigning value to life as a step function, where one nanosecond before it it's categorically not human and therefore disposable, yet one nanosecond afterwards it's a human and therefore killing it is immoral.

It may make practical sense to define laws that pretend that it's a step function, but it makes no moral reasoning sense.

I lean more towards the "early fetus is valuable" side of things, since I view future human life that hasn't been experienced yet as valuable. This is, after all, the main reason we care about rather nebulous existential risks - it'd be a shame if future humans never got the chance to exist. We therefore recognize that future human life has some value. Also, I'm well aware of the reductio ad absurdum that can be levied against my position, but I believe that that runs both ways.

Another point in support of this (rather horrific and terrifying) view is how babies were treated 200 years ago. Names were not given at birth, important religious events were postponed, etc. Until the parents had some reason to believe the baby will survive more than a year (which was not all that certain), they were not bestowing much humanity on the kid.

Not “who”, but “what” is a legitimate question.

Is an egg fertilised by only one parent and genetically modified to lack any higher brain structures (and therefore any function of mind) still a “human being”?

Can anyone who answers that question with “yes” justify it without reference to supernatural beliefs?

> "Those who look/think differently than me aren't human."

Sure, if by "think differently" you mean "don't think at all". There's a big difference between "eh, Jewish brains are not developed like ours" and actual rigor. I'm comfortable with the latter.

> Also, regardless of your political or moral beliefs on abortion, the unborn are certainly "human".

Well, I think you're manoeuvring around the point here. Clearly some combination of life and humanity is what matters, depending on what definitions people are glossing over at the time. Most people aren't upset by autopsies or cadaver work.

I think of my pet cat as a "person". Maybe you think that's crazy but I would suggest it's more person-like than a fetus, and that therefore I have a better definition of person than you are using.

At any rate the real issue is not who is a "person" but who is worthy of moral consideration.