One of the major philosophical arguments about veganism includes the argument from marginal cases. It attempts to show that you cannot believe both that humans have moral status and non-humans lack moral status with logical consistency.
The condensed version of the argument goes like this –
If human infants, senile people, the comatose, and cognitively disabled people have direct moral status, non-human animals must have a similar status, since there is no known morally relevant characteristic that those marginal-case humans have that animals lack.
For example, consider a cow. When we ask why it is acceptable to kill this cow for food – we might argue, let’s say, that the cow is an irrational being. However, many groups of people – infants, comatose – don’t embody rationality either. Hence, if rationality is accepted as a criterion, we must also accept killing kids in addition to killing cows. This contradicts the initial premise.
However, the argument itself is contradictory to the moral system (any) it is based on. In all three major traditions of morality – Utilitarianism, Deontology and Virtue Ethics – we take certain axioms to build the rest of it. A necessary assumption is establishing the rationality of man. Man is a rational animal, such is the truth that we accept. However, by this assertion, one doesn’t mean that every human is rational every time. Instead, we recognize that rationality is the nature of man or normality of humans.
The implication is that morally relevant traits (rationality for instance) aren’t dismissed solely due to outliers. Therefore, one can introduce the trait of rationality, as an example, to distinguish between humans and animals. To not accept would lead to reductio ad absurdum on the moral system applied.