This is an especially awkward question when it applies to human life. Rightly or wrongly, we consider our species more important than and dominant over others. See, for example, Aquinas, Animal Rights, And Christianity or Genesis 1 (verse 26) or Canadian Seal Hunt web site.
When it comes to human life, the question gets even tougher for us. It leads to intricate and sometimes fierce and vociferous debate, sometimes extended to physical attack or even murder. Imagine that: taking human life (by an anti-abortionist) when you're supposedly opposed to the very taking of human life. How irrational and contradictory we can be, how flawed!
I don't mull over such matters very often, preferring to spend my time on more pleasant matters. Strangely enough, I was led to this stream of painful thought when I took an unexpected detour while dallying in my comfort zone of technology and computing.
Perusing Wired Magazine's blog, I just came across McCain Equates Embryos and Fetuses in Stem Cell Statement (19 September 2008).
I encourage you to read the entire short article. A few brief extracts follow:
John McCain's recent statement on embryonic stem cell research was ambiguous in some ways, but clearly misleading in another: He equated human embryos with fetuses, and used language implying that farming fetuses for their tissues is a realistic possibility. ... Though the bill was unanimously approved in the House and Senate, its sponsors were criticized for failing to make clear that "fetal farming" doesn't exist.
Embryos used to produce embryonic stem cells are harvested after five days, when the embryo is still an undifferentiated blob of about 70 cells. While there is no sharp line for when an embryo becomes a fetus, nine weeks is a good rule of thumb; the industry standard for halting development on research embryos is two weeks. No reputable scientist has supported fetus experimentation. For McCain to revive the language of "fetal farming," say bioethicists, was misleading.
This was the first time that I've seen a specific number of cells specified, and a specific number of days or weeks of gestation. Why choose 70 cells rather than 80 or 100, and why is nine weeks a good rule of thumb?
Humanism aside, this is a complex minefield for bioethicists and legislators -- not to mention thise directly involved (medical practitioners, nurses, researchers, mothers, and so on). This is all very alien and uncomfortable for me, but I felt compelled to add such basic questions to this blog.