The marginal cases argument (MCA) has been used very effectively in defense of the direct moral standing of sentient non-humans. Any being that has direct moral standing is such that our treatment of them matters directly, independently of how it affects others. Direct moral standing is worth wanting: it ensures protection against, among other things, cruelty, mistreatment, destruction and exploitation, etc. Probably works of art and other possessions have no more than indirect moral standing; certainly normal adult humans have direct moral standing.
MCA’s are designed to undermine our confidence that the possession of a particular property R is necessary for direct moral standing. Well, it’s a little more complicated. Abbreviating a lot, MCA’s ask us to answer this question:
Q1. Are you more certain that having property R is necessary for direct moral
standing or that human being H (where H is a non-R) has direct moral
Let me instantiate the question so that it sounds more familiar.
Q2. Are you more certain that Rationality is necessary for direct moral
standing or that Suzie (say, a normal, 1yr. old, not-quite-rational human)
has direct moral standing?
Most thoughtful people seem more certain that Suzie has direct moral standing—that it matters directly what we do to Suzie, unlike what we do to chattel or to bits of property—than that rationality is necessary for direct moral standing. And, as it happens, there are counterparts of Suzie on the margins of most other properties allegedly necessary for direct moral standing. Most thoughtful people seem more certain that the counterparts of Suzie have direct moral standing than that those properties are necessary for direct moral standing. It appears—as most everyone knows—that the only rational way to ensure direct moral standing for all human beings is to concede direct moral standing to sentient non-humans as well. And so, many philosophers and others have concluded that we should expand the circle of beings that matter directly.
But the consistency requirement in MCA does not seem sufficient to determine which properties are necessary for direct moral standing. In answering question Q1 above we know far too much. We know who possesses the property and who doesn’t, so we can choose necessary properties in a way that favors the humans we care about and disfavors those we don’t. If rationality were necessary, some humans we care about would be excluded from the moral community, and similarly for autonomy, language-use, memory, self-consciousness, and so on and on. Therefore none are found to be necessary.
But there is a straightforward way to mitigate the influence of this information and make the marginal cases argument more impartial. Suppose you have to answer Q1 under these assumptions:
1. You do not know who possesses the property allegedly necessary for direct
2. You do not know who does not possess the property.
3. You do not know who is on the margins of the property.
Now suppose the question arises as to whether some early-term fetus or other (call it F) has direct moral standing or whether aborting F is perfectly permissible. It is typically argued that the answer to that question is clearly no, F has no direct moral standing, since F does not possess (or does not possess sufficiently) some property R that is necessary for direct moral standing. But whether this answer is correct depends on how we are prepared to answer Q3:
Q3. Are you more certain that property R is necessary for direct moral standing or that F (who is, for all you know, identical to you, or identical to a precursor of you) has direct moral standing?
The question in Q3 I think raises some interesting questions in both moral epistemology and moral methodology. Q3 assumes (more broadly than Rawls did) that you do not know where on the developmental scale you are. And of course it raises similar metaphysical worries. But Q3 points up that it’s easier to reach flimsy conclusions concerning which properties matter to direct moral standing when there is less at stake for us than it is when there’s much more.
But should theoretical conclusions be given more credence when they’re made with something at stake? Practical reason and theoretical reason intersect here and no doubt some epistemologists would urge that theoretical conclusions should be informed by such practical considerations.
For what its worth, there is almost no property R that I am so certain is necessary for direct moral standing that I am willing to bet my adult life on it. So I’d likely answer Q3 that I’m less sure that property R is necessary for direct moral standing than that F has direct moral standing.