Less Able Humans
This version of the Argument from Less Able Humans is a slight adaptation from the slides of a philosophy professor who has given permission to use them. He teaches this as an argument for the Principle of Equal Consideration, then allows that even if we weaken that substantially, the case for veganism follows from empirical reality.
Many carnists assume that, although we have strong moral reasons not to harm (and to benefit) sentient humans, we lack (at least in many contexts) strong moral reasons not to harm (or to benefit) sentient non-human animals. The most natural attempts to justify this assumption invoke certain intellectual abilities that typical human adults have and sentient non-human animals lack, such as abstract reasoning ability, the ability to think about ethics, the ability to enter into agreements, moral agency, etc. The basic idea of the argument from less able humans (aka "marginal cases") is that these justifications fail, because there are many sentient humans who lack these fancy intellectual abilities, but we have still have strong moral reasons not to harm (and to benefit) them. The only other potentially relevant difference between these intellectually less able humans and sentient non-human animals is bare biological species membership, but when we distinguish bare biological species membership from its typical psychological accompaniments, we can see that it is simply something like the ability to interbreed and produce fertile offspring, shared history of phylogenetic descent, or gross morphology. But these bare biological or historical features are as obviously irrelevant to our basic moral reasons not to harm or to benefit someone as other bare biological and historical features like her gamete size or chromosomal configuration (sex) or her skin color, hair texture, and facial features as a result of her ancestors' area of ancestry (ethnicity or race). So we really should conclude that, just as lesser intellectual ability does not make the well-being of intellectually less able humans in itself less morally important than ours, lesser intellectual ability also does not make the well-being of sentient non-human animals in itself less morally important than ours.
It is important to clarify what it means to hold that someone's well-being is not "in itself less morally important" than ours in virtue of her lesser intellectual abilities. This is not to deny that intellectual ability has derivative moral relevance, insofar as it affects how much a being might be harmed or benefited by a certain form of treatment, or indeed how much others might be affected by her condition (for instance if she is an emergency medic capable of saving others). Indeed, a certain minimal amount intellectual ability is likely to be precondition for being phenomenally conscious states (i.e. capable of having states that feel a certain way from the inside) or being sentient (capable of experiencing pleasure or enjoyment and pain or suffering) at all, which is a plausible necessary condition for being harmed or benefited at all. (This necessity of phenomenal consciousness or sentience for being harmed and benefited is supported by almost all leading theories of well-being, including hedonism, which holds that the only things that are beneficial or harmful in themselves and apart from their further effects are pleasure/enjoyment and pain/suffering; desire fulfillment theory, which holds that what is beneficial/harmful in itself is getting what we want/are averse to in itself; and most objective list theories, which agree with hedonism that pleasure and suffering are among the things that are good or bad in themselves, but add to the list of inherent good things such as friendship, achievement, and knowledge).
Moreover, intellectual ability might affect the extent to which one experiences inherently good or bad states as a result of certain things, and perhaps (in part because of this) how much one benefits from continued existence. Thus, although the view can certainly be challenged, many hold that intellectually less able humans and non-human animals inevitably benefit less from ordinary life than typical humans, and are thus harmed to a lesser extent by death. (Perhaps the most famous proponent of the argument from less able humans, Peter Singer, himself at least at one time held a view somewhat like this, according to which (roughly) death harms us by frustrating our current desires, so if intellectually disabled humans and infants lack future-directed desires, they are not harmed by death. This view is extremely dubious: there seems to be no good reason to think that someone is not harmed by missing out on things that she does not currently want if she will appreciate and benefit from them in the future, and it seems implausibly myopic to care only about someone's current desires and neglect the desires she will have in the future. But Singer's view illustrates just how much the view that intellectually less able beings' well-being is in itself just as morally important as that of intellectually more able beings can be consistent with the view that these beings benefit differently from continued existence). On such views it is consistent with an intellectually disabled child's well-being being in itself just as morally important as a typical human adult's for us to choose to save the adult over the child if we can only save one (or for us to, as is common in many jurisdictions, not invest the same resources to save the life of the a newborn who will be profoundly intellectually disabled as we would to save the life of an adult). Because on these views the adult would benefit more from living, we should, if we give equal importance to the well-being of the adult and the child, give more weight to this greater benefit to the adult, since to care just as much about what is (on these views) a lesser benefit to the child would be to treat the well-being of the child as though it had more moral importance in itself than that of the adult. Intellectual ability might also affect the extent to which others are harmed or benefited as a result of one being harmed or benefited. Thus, if an emergency medic can save the lives of two wounded children if but only if she survives, and we must choose between preventing the medic from being killed and saving the life of a third child, protecting the life of the medic is quite in line with treating her interests as no more morally important in itself than that of the third child (even assuming that they will benefit equally from continued existence). Considered in themselves, there is no reason to save the medic over the child, but because saving the medic in effect saves three individuals (her own and that of two others), equal inherent concern for the well-being of all four of the individuals requires that we save the medic (and save the three) rather than save the child (and save only one). What it is for someone's well-being to be in itself just as important as someone else's is for us to have equally strong moral reasons to not inflict (or to prevent) equally sized harms - e.g. equally intense amounts of suffering - on each of them, independent of their further effects.
With that in mind, we can formalize the argument from less able humans as follows:
The Argument from Less-Able Humans ("Marginal Cases")
P1. Some sentient humans (infants, young children, profoundly intellectually disabled) are intellectually comparable to sentient non-human animals
P2. If the well-being of sentient non-human animals (e.g. their avoiding a given amount of suffering, their benefiting from a given quality of life) is in itself less morally important than ours (in virtue of these lesser intellectual abilities), then the well-being of these sentient humans is in itself equally less morally important (in virtue of their lesser intellectual abilities)
P3. But the well-being of these sentient humans isn’t in itself less morally important than ours
Therefore, C1. The well-being of sentient non-human animals is not in itself less morally 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.
