And this isn't even about brain function. If a person is brain dead, indeed, we would consider them dead, but what if it was possible that the person, so injured, was capable of regenerating their brain if left on a machine long enough? Would we be so quick to pull the plug, even if the resulting personality was not the same as the person's who was injured?
Of course, humans cannot regenerate their brains after having died, but one class of humans does grow a brain for the first time. They may be brainless, but they are still human, and more than that, they are not simply potential humans, they are humans with potential, as much potential as anyone else would have.
You're right about this not just being about brain function. It's about sentience. From a strictly logical position, as long as there is a brain present, there is the potential for life. Then they have sentience. Embryonic stem cells are not brains nor are they brain cells. They are more close to any other cells of your body. Can you argue that a skin cell is a human being? What about a hair follicle? A heart? An arm? No? If these are not human, then what is? These all contain DNA that are unique.
If a human being was born or through some accident lost all their limbs and only had a torso, would they still be human? Why? Its because they can think and they can reason. They have a brain and they have sentience.
How then can embryonic cells, which are cells (albeit more complex than normal cells; but less complex than an organ) are classed as a human being?
This is not about classifying people as different levels of humanness, its about determine where the line is of what is human and what is not. The idea of "human-ness" beginning at conception is too vague.
If something is defined as human simply because it has the potential to become human, then sperm and eggs should also be human. And every woman who has a period and every man who has a wet dream are committing murder.
Some people are dealt really bad cards in life. I've certainly been lucky in many ways, so far, but I know full well that other people haven't. Still, taking the way even the potential for life from one person is not an acceptable means for redressing the inbalances in another person's life. The argument is the same for abortions as it is for stem cell research with embryos.
Leaving aside the argument of what is defined as a person, what right do healthy people who are not faced with these problems have to define what is acceptable to people who are afflicted? This question affects both people with diseases and people who need abortions. Its easy to stand on the sideline and say that it is wrong but in the end, the people on the sidelines are not the ones who suffer the consequences.
I know it's difficult to stand there and put up a stop sign when you feel that you could be helping people out that you can see everyday. But it's not cold and it's not cynical, it's what has to be done. As much as you want to help people who are in pain here and now, you still have to think about everyone else, today and tomorrow.
I think its irresponsible for people (not you in particular, people in general) not to make a stand for people alive now and rather make a stand for imaginary people (people who are not alive yet). The argument that "it is a slippery slope" or that "it will be the undoing of society" are not enough to justify not making the stand. It is more responsible to recognize that "yes, it is a difficult question, but we will deal with it after we save the people now and worry about the morality later".
I feel that humanity is not as stupid as people generally think. Yes, we've made mistakes but we're learning. If we turn away from this because we are afraid of what might happen or because we think its wrong (based on religion; and not based on science) then we are not fulfilling our potential. Hundreds of years ago, people thought the world was flat and to think otherwise was heresy and invited persecution. Now, people refuse to pursue a science that could cure so much diseases, and those who do are branded criminals.
To not face difficult questions or make difficult choices is not what it means to be human.