Before I begin the actual discussion, I want to take a moment to clarify and/or debunk few of the obvious equivocations which might be made here. First, the equivocation that a corpse is also a human, but not a person--and that the corpse as such has no rights of its own. Ok, but when referring to a corpse, we usually say that it is a dead "person," not a dead "human," so the analogy really backfires here. Moreover, a corpse is not equivalent to an unborn child, because the unborn child is alive, whereas the corpse is dead.
Second is the complaint that a piece of tissue is fully human, and yet not a person. This argument is flawed in much the same way as the corpse argument: the piece of tissue will not survive when isolated from the rest of the body, no matter what kind of life-support measures are undertaken. The unborn child is often viable without any kind of life support; even in the first trimester, the unborn child is a complete entity, not only with his own DNA, but also with the ability to survive and grow when separated from the mother (as IVF treatments have demonstrated). The child would quite probably be capable of growing entirely outside of his mother, if given the proper life-support.
Now, these two points for equivocation aside, the main issue at stake is, "what is the difference between a human and a person." The science-fiction nerd/philosopher in me would say, tongue-in-cheek, that humans are a sub-set of all beings who could be called "person." This is certainly true theologically, as God is a Person (Three, in fact), as are the angels and the demons. I might add that if there is intelligent life elsewhere in the universe, its members would also probably qualify as "people."
The argument concerning who is an who isn't a person is, in the end, a metaphysical one. It is based on reality, but on that part of reality which can't be accessed by science. Science can tell us basic fats--such as that an unborn child is biologically human and is in fact a separate human from his mother--but it is dumb as concerns metaphysics. The definitions for when a human "becomes" a person are fairly varied, and science might help determine when a specific criterion is met; it cannot prove the veracity of the claim that this is the defining moment of transition from merely "biologically human" to "human person."
There are several definitions of when a human might become a person, one of which is "at conception." Any contrary claim relies on some concept of "ensoulment," a controversy discussed by many of the greatest mind in Christendom--including Saints Thomas Aquinas and Augustine*. And while we can use science to develop technologies which can detect or otherwise measure brain waves, heart beats, emotions, and when the child feels pain/pleasure, science tells us nothing about whether or not a soul becomes present in the child during any of those times. In the words of John C. Wright
"Whenever the debate turns to abortion, it is the anti-abortionists who talk science, and the pro-abortionists who talk metaphysics....The anti-abortionist says a baby in the womb is human if it has the genetic characteristics of a human organism; the pro-abortionists say the baby is not human until after ensoulment--even though they use another word than ensoulment, they are nonetheless talking about an invisible metaphysical property that makes a human organism a human being in the legal and in the moral sense, an object to which human dignity and spiritual properties, such as moral worth, attach: A being with a soul.The argument for abortion which is based on these concepts of ensoulment is further flawed by the fact that our legal system for the most part does not rely on whether one believes that anybody else has a soul or not. Materialists do not believe that anybody has a soul, and ultimately argue that these things which we relegate to the realm of metaphysics--souls and spirits, intelligence, the mind, emotions, etc.--are merely "emergent phenomena." These things exist only because the physical reality exists; thus, by this system of thought, it is impossible to determine the non-existence of the metaphysical attributes of personhood in an unborn child (or, alternatively, it is impossible to determine the existence of such attributes in a twenty-five year old man). Yet, the materialist is expected to obey the same prohibitions against murder as any other person. You don't have to believe in the soul either way to know, deep down, that murder is wrong, and that it is a bad idea for society to allow it.
Some moderate pro-aborts want to limit abortion onto to babies before they develop a nervous system. The pre-brain baby is not ensouled, in their [metaphysics], and the post-brain baby is ensouled. Of course, as secular materialists, they are unable to answer why a post-brain baby who [is] in the womb, and has no more capacity to reason than a churchmouse or a housecat, should be given the dignity of human rights. Perhaps it is only the potential for intelligence they revere. It is never clear to me why a person's humanity is an insufficient warrant to treat him as a human being, but his potential for intelligence is a sufficient warrant for treating him as a human being, especially when the distinction is being made between a blastulae that will one day develop a brain, and a fetus that has a brain or a proto-brain but no conscience or rational thoughts, no speech or moral decisions, taking place inside it.
As ever, on the topic of abortion, it is so-called mystics [pro-lifers, particularly religious ones] who talk science and common sense, and the so-called scientific types who talk mysticism, paradox, and nonsense....And they call us anti-science."
Moreover, the fact that the stage at which ensoulment occurs is a speculation means that the prudent course of the law is to assume that it occurs at the earliest time possible (conception), not the latest (sometime well after birth, by the account of some). Thus, the argument of uncertainty best applies as a pro-life argument, not a pro-choice one. As for the argument in which humanity is separated from personhood, well, few if any good things have ever come from that mode of thinking; rather, it has given us many of the horrors of human history, from the Nazi mistreatment of the Jews to slavery to human sacrifices (in civilizations such as the Aztecs or the Phoenicians). Applying this distinction to abortion is no different.
*It is worth noting here that although these great saints lived in an era before ultrasound technology was available, and thus formulated interesting opinions of when after conception (though before birth) ensoulment occurs, both opposed abortion at any stage if for no other reason than that the Church opposed it. This is something which the so-called "pro-choice Catholics" ought to keep in mind when citing such thinkers to attempt to make a "Catholic Case for abortion."
If you found this post helpful, some related posts may be found here:
The Shame of Silence: Men and Abortion
35 Years of Roe and Doe
Speaking Up, If Painfully
Some Thoughts on Abortion: Some Rights are Greater than Others
Tactics for Avoiding a Terrible Fact: "Famous Violinist"
Righteous Fear of the Lord and the Pro-Life Movement