The argument, in essence, is that we have, over centuries of history, expanded the circle of beings whom we regard as morally significant. If you go back in time you'll find tribes that were essentially only concerned with their own tribal members. If you were a member of another tribe, you could be killed with impunity. When we got beyond that there were still boundaries to our moral sphere, but these were based on nationality, or race, or religious belief. Anyone outside those boundaries didn't count.
Slavery is the best example here. If you were not a member of the European race, if you were African, specifically, you could be enslaved. So we got beyond that. We have expanded the circle beyond our own race and we reject as wrongful the idea that something like race or religion or gender can be a basis for claiming another being's interests count less than our own.So the argument is that this is also an arbitrary stopping place; it's also a form of discrimination, which I call "speciesism," that has parallels with racism. I am not saying it's identical, but in both cases you have this group that has power over the outsiders, and develops an ideology that says, Those outside our circle don't matter, and therefore we can make use of them for our own convenience.
That is what we have done, and still do, with other species. They're effectively things; they're property that we can own, buy and sell. We use them as is convenient and we keep them in ways that suit us best, producing products we want at the cheapest prices. So my argument is simply that this is wrong, this is not justifiable if we want to defend the idea of human equality against those who have a narrower definition. I don't think we can say that somehow we, as humans, are the sole repository of all moral value, and that all beings beyond our species don't matter. I think they do matter, and we need to expand our moral consideration to take that into account."
—Peter Singer, author and ethicist, in a 2006 interview with Salon.