(1) It is prima facie wrong to kill human beings.
(2) Fetuses are human beings.
Therefore, it is prima facie wrong to kill fetuses.
Warren claims this argument commits the fallacy of equivocation:
"the term `human' has two distinct, but not often distinguished, senses. This fact results in a slide of meaning, which serves to conceal the fallaciousness of the [above] argument ... For if `human' is used in the same sense in both (1) and (2) then, whichever of the two senses is meant, one of these premises is question-begging. And if it is used in two different senses then of course the conclusion doesn't follow.Warren does not explicitly call 'human' ambiguous between 'person' (= 'full-fledged member of the moral community') and 'Homo Sapiens' (what she thinks of as 'having a genotype within a certain range,' though that's not how biologists think about species), but I don't think it's too much of a stretch to attribute that view to her.
Thus, (1) is a self-evident moral truth,' and avoids begging the question about abortion, only if `human being' is used to mean something like `a full-fledged member of the moral community.' (It may or may not also be meant to refer exclusively to members of the species Homo sapiens.) We may call this the moral sense of `human.' It is not to be confused with what we call the genetic sense, i.e., the sense in which any member of the species is a human being, and no member of any other species could be. If (1) is acceptable only if the moral sense is intended, (2) is non-question-begging only if what is intended is the genetic sense."
The pedestrian question I want to ask is: is the word 'human' really ambiguous between 'person' and 'Homo Sapiens'? Linguists have developed tests to determine whether a word is ambiguous or not, and I'm not sure 'human' comes out ambiguous on these diagnostic tests. Here are three tests linguists consider useful.
1. The other languages test. Do other languages have distinct words for the various meanings of the supposedly ambiguous word?
2. The unrelated antonyms test. Does a word have two unrelated antonyms? E.g. the ambiguous word 'light' has both 'heavy' and 'dark' as antonyms.
3. The conjunction reduction test. Consider the sentence 'John and Jane each have a bat.' This could mean they both have baseball bats, or it could mean they both keep flying mammals as pets. However, it cannot (ordinarily/ without punning) mean that John keeps a pet bat and Jane has a baseball bat. That is, so-called 'crossed readings' are impossible, if the word is ambiguous. (This is called the 'conjunction reduction test' for the following reason: if John keeps a pet bat (but has no baseball bat), 'John has a bat' is true. If Jane has a baseball bat (but has no pet bat), then 'Jane has a bat' is true. But the reduced conjunction sentence 'John and Jane both have bats' is untrue, unless you are punning/ joking.)
The question now is how 'human' fares on each of these tests.
(Before proceeding to the official tests, it might be worth noting that, at least for me, 'human' does not intuitively/ pre-theoretically feel similar to 'bank' or 'light'.)
1. (other languages) Though I think 'human' apparently comes out unambiguous on this test for the languages I know, I don't know enough languages to be comfortable making a definitive pronouncement about this.
2. (unrelated antonyms) I think 'human' fails this ambiguity test too -- though I am open to evidence to the contrary. (hmmm... How unrelated do the antonyms have to be?)
3. (conjunction reduction) Suppose someone is 4 weeks pregnant, and she decides to name the fetus 'Pat'. Further suppose that we are at some point in the future where aliens are full-fledged members of our moral community. Call this alien 'Gordon Shumway.' (If you prefer robots to aliens, that would work too.)
For me, we can't even get the ambiguity test off the ground: 'Gordon Shumway is (a) human' (or 'Johnny Five is (a) human') are both false, according to my semantic intuitions. And if there is no true reading of 'Gordon Shumway is human,' then thinking about 'Pat and Gordon Shumway are human' won't reveal anything.
The (purported) fact that 'Gordon Shumway is human' is false suggests that belonging to the species Homo Sapiens is a necessary (but perhaps not sufficient) condition for being human. If that is correct, then we should not say that the traditional anti-abortion argument above trades on an ambiguity in 'human': there is no purely 'moral sense' of the word 'human' (i.e. full standing in the moral community is insufficient for humanity).
However, this is not a real problem for Warren's criticism of the traditional anti-abortion argument, as long as we think that "It is prima facie wrong to kill any member of Homo Sapiens" stands in need of some justification -- i.e. we think that we need some justification to think that the biological facts of species membership have anything to do with moral rights and obligations. And how to argue from biological premises to moral conclusions has been an extremely contentious philosophical issue.