Stephen Law writes about Speciesism, Potential and Normality, investigating the ethics of eating animals. Unfortunately, he takes an ass-backward approach to the analysis, which merely confuses, rather than clarifies the issue.
The fundamental issue is the attempt "to justify our discriminating between pigs on the one hand and equally dim humans on the other by appealing to the notions of potential and normality."
The argument that some elaborate justification is even necessary is fallacious: if we have the right to eat pigs because they are unintelligent, then we have the right to eat equally unintelligent human beings. However, this argument works only if we add "only" to the justification: if we have the right to eat pigs only because they are unintelligent, then we have the right to eat equally unintelligent human beings. From a purely logical standpoint, we can account for our moral intuitions with a complex principle: we have the right to eat pigs because they are unintelligent and not human beings.
Obviously, this complex justification relies on raw speciesism: We should not eat members of our own species just because they are members of our own species. Law is not at all stupid, and he attempts to undermine this complex justification by comparing speciesism to racism and sexism. The justification is, if race and sex are inadequate basis for ethical discrimination, then speciesism requires a separate, principled justification. In Law's own words,
The challenge facing those of us who believe it is morally acceptable to kill and eat, experiment upon, etc. other species but not our own is, in effect, to point out the difference between us and them that morally justifies this marked difference in treatment. Unless we can point up some such difference, it seems that we, too, are guilty of a form of bigotry: the form for which Richard Ryder coined the term speciesism.
But why should we require any deep principled justification? We can certainly point out that pigs and humans are indeed different species. Law therefore endorses Ryder's assertion that we should point out a difference other than species.
Ryder's argument is also subject to a subtle form of infinite regress. Suppose we could find some difference X between pigs and humans. It would then be just as arbitrary to make an ethical discrimination on the basis of X as it would be to make an ethical discrimination on the basis of species. A clever philosopher would just say that The challenge facing those of us who believe it is morally acceptable to kill and eat, experiment upon, etc. beings with X but not beings without X is, in effect, to point out the difference between beings with X and beings without X that morally justifies this marked difference in treatment. Unless we can point up some such difference, it seems that we, too, are guilty of a form of bigotry: the form for which I might coin the term Xism.
Furthermore, there are substantive differences between racism and sexism on the one hand, and speciesism on the other. First, racism and sexism have typically been used to deny ordinary rights, whereas speciesism in this context is used to assert extra rights, only for a small minority of human beings without ordinary human intelligence. Granting extra privilege on non-principled grounds is a much easier sell than denying ordinary rights. I have an ethical obligation to provide medical treatment to my pet cat, but no corresponding obligation to provide the same treatment to every stray cat, even though there is no principled difference between my pet cat and the neighborhood strays. I assume the obligation -- and when I coughed up $1,200 to treat my cat, you can be sure the concept of ethical obligation was strong in my mind -- just because I happen to care more about one particular cat than another.
Secondly, there are sapient, intelligent beings who can and do object to being denied their own ordinary rights on the basis of race and sex. However, all beings capable of objecting to speciesism are those privileged by the principle; the beings denied this privilege, i.e. animals, are not capable of objecting. On the ethical basis of conflict resolution, there is no conflict whatsoever between human beings and those actually denied some privileges.
The attempt is futile and misguided to reduce ethical obligations to absolute universal principles, especially simple principles without conjunctions.
On a purely evidentiary basis, all we can do is attempt to explain human moral intuitions. Such an endeavor is worthwhile, but it's not philosophy: It's purely descriptive individual and social psychology.
On a deductivist basis, we're stuck with deductivism's foundational problems: how do we justify our axioms? "Sapientism" -- the idea that it's permissible to eat non-sapient beings and impermissible to eat sapient beings -- is just as arbitrary and unjustified as speciesism. We might just as well say that without some principled justification for sapientism, it is just as bigoted as speciesism -- or even racism and sexism. And even if we did fine some principled justification for sapientism, we could attack that principle on the same grounds.