Marginal Cases
The Argument from Less-Able Humans ("Marginal Cases")
P1. Some humans (infants, young children, profoundly intellectually disabled) are intellectually comparable to non-human animals
P2. If the well-being of non-human animals (e.g. their avoiding a given amount of suffering, their benefiting from a given quality of life) is morally less important than ours (in virtue of these lesser intellectual abilities), then the well-being of these humans is equally less important (in virtue of their lesser intellectual abilities)
P3. But the well-being of these humans isn’t morally less important than ours
Therefore, C1. The well-being of non-human animals is not morally less important than ours
This entails (if you like in conjunction with P4. Our well-being is morally important) the Principle of Equal Consideration: human and non-human animal well-being is of equal intrinsic moral importance (i..e moral importance in itself and apart from its further effects) - e.g. all else held equal, the fact that an act would inflict a given amount of harm (e.g. a given amount of suffering) on a human or a non-human animal is an equally strong moral reason against it.
Defense of P3: It is deeply implausible that intellectual ability affects the intrinsic importance of one's well-being once we distinguish (i) its role in making one a moral agent who owes duties vs. a moral patient who is owed duties, (ii) its role in affecting the instrumental importance of one's well-being for others, and (iii) its role in determining how beneficial or harmful certain things are for you (including how much typical human adults benefit from living vs. how much non-human animals and profoundly intellectually disabled humans benefit from living).
Defense of P2: The only relevant thing that distinguishes non-human animals from intellectually comparable humans is bare biological species membership, but it's deeply implausible that bare biological species membership is relevant to the intrinsic moral importance of someone's well-being once one we focus on what it really is: something like potential to interbreed to produce fertile offspring, psychology-independent morphology, phenotype-independent genotype, history of phylogenetic descent. It's no more plausible that these matter to the intrinsic moral importance of someone's well-being than someone's ethnicity / continent of ancestry and consequent facial features, hair texture, and skin colour (race), or her chromosomes and relative gamete size (sex).
The weakening: Even if somehow intellectual ability or biological species memebership per se mattered to the moral importance of someone's well-being they couldn't matter very much. Since they seem utterly devoid of moral importance; surely it is safe to at least conclude:
C2. Principle of Minimal Consideration: We should / are morally required to avoid inflicting enormous harm on non-human animals for what is at most relatively trivial benefits for ourselves.
Empirical considerations about factory farming, human health, environmental effects, and, if you like, further philosophical considerations about what makes death a harm, the potential relevance of the fact that future farmed animals won't exist unless we buy animal products, and the probabilities that one's purchasing decisions will make a difference of various kinds and to what extent this matters, we get:
P5. To avoid inflicting enormous harm on non-human animals for what is at most relatively trivial benefits for ourselves, we must be vegan.
Finally, C2 and P5 entail:
C3. We should / are morally required to be vegan.
References
Degrazia, David. 1996. Taking Animals Seriously: Mental Life and Moral Status. Cambridge: Cambridge University Press.
Engel, Mylan. 2000. The Immorality of Eating Meat. In Louis Pojman ed., The Moral Life: An Introductory Reader in Ethics and Literature, New York: Oxford University Press.
McMahan, Jeff. 2002. The Ethics of Killing: Problems at the Margins of Life. New York: Oxford University Press.
Norcross, Alastair. 2004. Puppies, Pigs, and People: Eating Meat and Marginal Cases. Philosophical Perspectives 18: 229-245.
Singer, Peter. 1974. All Animals Are Equal. Philosophical Exchange 1: 103–16.
Singer, Peter. 1975. Animal Liberation. New York: Harper Collins.
Singer, Peter and Jim Mason. 2006. The Ethics of What We Eat: Why Our Food Choices Matter. New York: Rodale Press.
