What is a person, anyway?

Many moral debates hinge on the concept of person. Arguments against abortion, for example, often involve the claim that fetuses are people. Here's a common argument for the conclusion that abortions are immoral.

(1) It is wrong to kill an innocent person.
(2) A fetus is an innocent person.
(3) Therefore, it's wrong to kill a fetus.

Assessing the soundness of this argument is difficult. There's much to be said about the first premise and about related issues involving the notion of a right to life. But we can save these issues for another day. What I'm concerned about here is the second premise and how one is to determine what it is to be a person. Mississippians are set to vote on a measure that involves settling on a legal definition of personhood. This vote could have a tremendous impact on various policies, not just those related to the legal status of abortions in that state.

So what are the criteria for personhood exactly? I am a person but my dog is presumably not. What are the relevant properties that morally distinguish us? It better not simply be that I'm a member of the species homo sapiens or that I have a higher IQ than him. This is because species membership and intelligence (like race, religion, gender, or taste in music) are not markers of moral status. These seem to be morally irrelevant properties. I don't care much for Nickelback, but I certainly don't think fans of the band have a lower moral status because of their taste--or lack thereof--in music. What we need to find are morally relevant properties that I have but which my dog lacks. These would be the properties relevant to personhood. Once we discover what these properties are exactly, we can then see if fetuses have them. And this is what we need to do if we are to answer the question of whether fetuses are people (in the moral sense of the term where this means that fetuses have a right to life or are deserving of deep moral consideration).

I'm curious what you find to be the morally relevant properties required for personhood.

UPDATE 11/9/11: The votes from Mississippi are in and Measure 26 did not pass. This indicates that the majority of voters did not feel comfortable with legally defining fetuses as persons. It may be that the rather strong implications of passing the measure are what dissuaded many voters. The measure was likely to effect more than just the practice of abortions in Mississippi. It was likely to have an impact on certain forms of birth control, fertility treatments like in vitro fertilization, etc. It's not clear to me what conclusions one ought to draw from this vote. Polls strongly indicate that the majority of folks in Mississippi are anti-abortionists and think that fetuses are people. But the vote indicates that, despite this position on the status of fetuses, most Mississippians (that voted) are not prepared to enact the policies involved in the measure. This is fascinating.

UPDATE 12/19/11: A preliminary poll on this blog regarding this issue resulted in 31 votes for considering a fetus a person and 46 votes for not.