Good point in your first paragraph. As a response, it is coherent to say that while something is always wrong, it may be the lesser of two evils in certain circumstances. Second paragraph is incorrect, another animal's zygote would not develop into a human if placed in the womb, so they are quite different intrinsically, even if they may appear the same.

@yters - Good grief, what do you think I mean by "developmental potential"?

@Rex - If you think there are significant intrinsic differences, as I point out, then you contradict yourself saying these differences are not notable. Otherwise, you mean something quite different with the phrase "developmental potential."

@yters - The point is that the things which make humans specially worth caring about has not yet happened in a two day old human zygote. Potential only gets you so far.

@Rex the point is the difference is not potential, it is actual, i.e. the DNA and other properties of the zygote that differentiate it from other species. Now, if you are saying humans should only be valued based on their usefulness to others, that's an entirely different argument.

@yters - Typical arguments for the unique sanctity of human life outside of a religious context (as asked for here) rely upon some actual, not potential, capability or experience pertaining to humans. For instance, humans are self-conscious (at least to an extent far surpassing other animals), and pretty much all of Kant's work presupposes a self-conscious actor. A zygote is not self-conscious, so that strategy doesn't work. It can't "suffer" any more than mildew in your bathroom can. You don't ascribe priviledged status to a potato you are about to eat because of its potential.

@yters - So you are left with things like "we have strong feelings for babies because our species would go extinct if we didn't, and we are so clever we know that this ball of barely-undifferentiated cells will become a baby, so we decide to apply the same standards to it"--which is not much of an argument since the same form leads to all sorts of horrific prescriptions (e.g. "we naturally favor opposite-sex attraction so homosexuality is immoral", "we naturally favor our own tribe so war and genocide are okay", etc.). It makes more sense to act as if humans have souls if you believe they do.

@Rex - there are good arguments for a soul outside of a religious context. See Plato and Aristotle. A modern take would be Thomas Nagel. At any rate, now you're onto a different question regarding the secular definition of personhood. Definitely a worthwhile question, and one I'm considering opening since another has expressed interest. Regarding potential, you are confusing the term "potential" with "undeveloped". Potential entails multiple possibilities. A more accurate term to describe the zygote is "undeveloped".

@yters - I suppose whether one views such arguments as "good" depends on whether one accepts the findings of the scientific revolution. Also, I am not sure what an "undeveloped experience" is, so I think your wording quibble is unhelpful. Call it what you will--it hasn't happened yet, it is not certain to, and one or both of those usually makes a big difference to us. To do otherwise in this case requires a good argument; I am not aware of one which does not look scientifically foolish (or I would have included it in my answer).

@Rex - Well, consciousness will develop provided the normal course of events is not stopped. It's like (though not identical to) someone being asleep and saying it's alright to kill them because they haven't woken up yet, and aren't certain to wake up. Of course, there is an enormous physiological difference between the two cases. However, in both cases there is a biological process in motion that will bring about consciousness at a certain point. Now, before conception this biological process is not in motion, but potentially exists. Hopefully this clarifies my distinction.

@Rex - I created a question whether there are secular arguments for the soul: philosophy.stackexchange.com/questions/14013/…

@yters - The differences with sleep are pretty important, e.g. you have an actual conscious being (transiently not conscious, but with state stored) vs some cells that will eventually develop into a conscious being if all goes normally. The potato on my plate will also turn into part of a conscious being if all goes normally (e.g. I eat it). Until it's part of me, we don't treat it differently than any other possession of mine (e.g. my fork).

@Rex - Heh, well now I know you aren't being serious if you think a potato is equivalent to a zygote :)

@yters - Indeed I don't, which is why I reject simple notions of this-will-become-part-of-a-conscious-being as a useful way to ground our treatment of zygotes. "It will happen if you don't mess with it" is insufficient reason to not mess with it.

Well, in both cases of sleeping and zygote we have physical state that transition to consciousness...

Now, with a potato, you do not have a physical state that transitions to consciousness...

And, whether it-will-happen is sufficient reason or not (hopefully it is if sleeping is biologically equivalent!), the point is that it-will-happen most certainly distinguishes a zygote from a potato, and a human zygote from a frog zygote...

A question for you, why is it-will-happen not sufficient reason, without begging the question?

Of course you have, in a potato, a physical state that transitions to consciousness! When the potato is not eaten by me, it is not conscious. When it is, some fraction of it is incorporated into my body and then is every bit as conscious as any other potato's-worth fraction of my body (or brain or whatever you wish to identify with consciousness).

So a potato that I am about to eat with high reliability (let's say it's on my fork--I believe the chances then are around 99%) has the same "to-be-conscious-if-nothing-messes-it-up" status as does a blastula with normal chromosomes.

So the reason it-will-happen is not sufficient reason is that the actual, current state of things is important to us.

And the reason for that is that there very often is some substitute that will work perfectly well by the time we need it to. If I lose my potato, I can eat another (or something else). If I lose a blastula--it happens all the time with spontaneous miscarriages--well, if my wife and I were hoping to have a child it is disappointing because it means that we are not, but we still haven't had one yet so we can try again.

The only thing that is different about a zygote is that information stored within it is crucially important in the fate of that zygote (not only--the mother is also crucially important).

So one could decide that, for instance, any complex information-rich entity that will by virtue of usage of its stored information become conscious eventually with reasonably high reliability deserves the same attitude as a presently conscious entity.

One could decide that, but I'm not sure what justification one could give except as a rationalization for extending our instinct to protect our children as early as we can identify an ongoing causal process.

And that's why I object to the move from "human life is intrinsically valuable" to "human life begins at fertilization". A fertilized zygote does not have the qualities that we normally say that we value about human life (except inasmuch as it is necessary to continue human life).

Does this make things any clearer (or more convincing) @yters ?

@RexKerr Yes, it is quite clear what you think, and I'm glad you note the zygote is crucially different from a potato in the information it contains.

@RexKerr Now, it is hard for me to make sense of the claim that human life does not begin at fertilization. The fertilized egg is most definitely alive and most definitely human...This has always confused me regarding those who argue for abortion on this point. It is just not logical to say a fertilized egg is not a human life. From there, the pro-life argument is very straight forward, granted you also believe human life is intrinsically valuable.

@RexKerr One final point. It seems to me that the pro-life position is actually more properly scientific than religious. Religions, including Christianity, have historically believed the soul did not enter the body until 7 weeks and onward in the pregnancy. The modern pro-life position that a human life begins at conception is largely driven by the scientific recognition that a human is defined by the DNA formed at conception.

You don't say that human life does not begin at fertilization, nor do you say that human life is not intrinsically valuable (though I suppose one can argue the latter point). But you do notice that you are using different definitions of "human life".