Chris, a brain-dead human being is a “human organism”, and he is most certainly alive. But no one with any sanity objects to turning off his life support machines and thus killing him, because everyone recognizes that he has no consciousness and is therefore not—by any conceivable stretch—a person. In fact, he is far less of a person than a conscious animal is. And the same thing is true of an embryo—or an early-stage fetus—which has not yet developed any functioning brain cells, or has not yet started growing the interconnections between them in its cerebral cortex (which doesn’t even begin to happen until the 5th month). Therefore it is not only wrong, but downright ridiculous, to claim that there is ANY risk that we are killing a human person when we kill an embryo, or a first-trimester fetus. Not a one-in-a-million risk, or a one-in-a-billion risk. NO risk. And this is not fancy logic-chopping to try to justify abortions; it is plain, simple, common-sense physical fact.
Anti-abortionists and opponents of stem-cell research sometimes argue that, by killing embryos or first-trimester fetuses, we are keeping “potential” human persons from coming into existence. But by that same reasoning, any woman who doesn’t stay constantly pregnant is immoral—just think how many potential people SHE’S keeping from coming into existence!
But this is not Bruce's position; instead, he argues that the brain-dead human being is dead because s/he is is not conscious. I imagine that Bruce would probably reconsider, if he noted that there are all sorts of people who are unconscious yet not dead: the comatose (reversible or not), as well as the sleeping! Lack of consciousness -- temporary or permanent -- therefore cannot be the criterion for human dignity and its concommitant rights.
In the remainder of Bruce's first paragraph lies a somewhat-hidden distinction, between a human organism and a human person. Bruce apparently holds that not all (living) human organisms are human persons, and this is a fairly common position among those who deny the dignity of the embryonic human being. But the criteria which he offers are problematic, as seen above: the brain simply isn't necessary in the very young human being (the function it provides is already present), and consciousness is obviously not required for human dignity.
In his concluding paragraph, Bruce makes a claim which is also very common, and frankly, demonstrates to me that those who deny the dignity of the embryonic human do not read opposing materials very closely. Bruce thinks that people like me hold that killing a human embryo kills a potential person (he actually says that he thinks we hold that killing an embryo prevents a potential person from coming into existence, but I think he meant it as I stated it here). This is not so: the "pro-life" position holds that killing an embryo kills an actual person, not a potential person. An actual person came to exist at the moment of conception (or twinning or cloning), when the human organism came to exist. While I grant the distinction between human organism and human person, I also recognize that there is no human organism that is not a human person. And hence Bruce's attempt at an argument ad absurdum is rendered invalid: I oppose abortion and embryonic stem cell research because they entail the killing of a human being, which is not the case if a woman does not become pregnant.