David Sklansky:

In this thread, you maintain that abortion is murder, since the embryo is a person. You are the second intelligent atheist I've seen make this claim, yet when questioned as to how you came to this conclusion, there are hardly any replies, and the ones that are given seem to skirt the question.

I would like to know why you think an embryo is a person? What characteristics/criteria denote personhood (and, presumeably, how does an embryo have those)? Lastly, what criteria do you think medical science should use to denote when personhood ends (ie: death), and what objections would you have in using that same criteria to denote when personhood beings? (That last question is most important.)

We have discussed this in some depth in this thread, and I introduced some definitions that I'd like to use in order that we might not misunderstand each other:

hu·man
n.
A member of the genus H0mo and especially of the species H. sapiens.

adj.
Of, relating to, or characteristic of humans

life | a·live
n. / adj.
The property or quality that distinguishes living organisms from dead organisms and inanimate matter, manifested in functions such as metabolism, growth, reproduction, and response to stimuli or adaptation to the environment originating from within the organism.

per·son
n.
1. A living human.
2. The composite of characteristics that make up an individual personality; the self.
3. A human or organization with legal rights and duties.

So, to summarize:

Anything with human DNA is "human" (by the 2nd definition). Any living cell is "alive". A zygote, therefore, is a living human cell. A person is a living human with the right to life.