The marginal cases argument (MCA) is designed to undermine our confidence that the possession of a particular property R is necessary for direct moral standing. The argument asks us to answer this question:
(1) 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
The property involved is typically rationality or language-use or self-consciousness or awareness of the future, etc. For any such property R that we select as necessary for direct moral standing, there is some non-human, sentient being that possesses R to a greater degree than some human does. We are embarrassed into admitting that favoring humans over non-humans on the basis of R displays our bias for human beings. But MCA does not go far enough in eliminating bias.
Suppose you had to decide whether R is necessary for direct moral standing under the following conditions:
(2) You do not know where on the developmental scale you stand with respect to any
property R proposed as necessary to direct moral standing.
For all you know, you are an early term fetus–that is, for all you know, this is the current stage in your development. But you might be at the stage of a newborn or a normal adult human being or an aged adult. You simply have no idea. Now take any property R–any property that you might not possess at all or might possess to some greater degree. Here’s the question we want to ask:
(3) Are you certain enough that R is necessary to direct moral standing that you are
prepared to bet your entire adult life on it?
For my part, there is no property R that I’m so sure is necessary for direct moral standing that I’d be willing to bet my entire adult life on it. But that’s true for everyone. So when we insist that we know that some property R is necessary for direct moral standing, we are really expressing a bias arising from our knowledge that we are normal adults already. It’s not a bias for humans over other species, it is a bias for humans at a certain stage of development over those at lesser stages. You’re not a specieist, but you are a developmentalist.