I think that growing up reading fantasy and science fiction helped me to something important that I think most people in our culture don't understand: "human" and "person" are not synonyms. Our culture teaches us that certain qualities -- most importantly, to me at least, the ability to think and communicate abstract thoughts and to make ethical judgments -- are "uniquely human" and separate us from animals. In the "real world" that our culture is aware of, these qualities are unique to humans, but science fiction and fantasy allowed me to meet and know, if only in imagination, a whole host of creatures who clearly aren't human, yet have these qualities. Whatever it was that made me better and more important in the world than animals, Spock and Mnementh and Reepicheep and Mike from The Moon Is a Harsh Mistress had it too; they were people in my book.
Thus formed the first, and most important, idea in this piece: "human" and "person" aren't the same. A being can be a person without being human.
Since our culture tends to use "human" and "person" interchangeably without thinking about it, this led me to consider which of the two we really mean (or at least, should really mean) when we make value judgments and moral rules and laws. And the conclusion I came to was that pretty much everything that counts is tied to "person", not to "human", in my world view. "Human rights" should be "person rights". Murder is the unjustified killing of a person, not of a human. "Human dignity" belongs to all persons, whether human or not. "Human" is just a matter of genetics, and in a world where there are other kinds of people, not really very important.
So far, I've probably not said anything that will really offend a lot of people. I've split the concepts of "human" and "person", based on fictional examples who are persons but not human. And while there are humans who wouldn't agree, I suspect most people who'll read this journal would be with me in thinking that if we did contact the galactic civilization tomorrow, we would be obliged to give those non-humans who came to live among us the same special treatment we give our fellow humans.
Now is when I blithely ignore the "thin ice" warning and get to the controversial part. Having separated "human" and "person" into distinct categories by identifying "persons" who are not "human", my relentlessly logical mind has to ask if there are "humans" who are not "persons". And I find that there clearly are. Two easy examples are end-stage Alzheimer's patients and unborn babies. And that's why I find abortion and euthanasia both acceptable. Both are troubling, for we should not kill any living thing lightly or without good reason, but neither is murder.
I recognize that allowing the idea of humans who are not persons creates grey areas where it will be hard to draw the line, but just because it doesn't lead to easy answers doesn't mean that the premise is wrong. (In fact, a premise that does lead to easy answers to ethical questions is suspect at best, but that's another topic.) I also recognize that there are some humans who justify great evil by a superficially similar rationale that certain groups of humans aren't really people, but just because evil people have espoused an idea doesn't make the idea wrong.