"In contrast with this approach, the view that I want to defend puts human and nonhuman animals, as such, on the same moral footing. That is the sense in which I argued, in Animal Liberation, that "all animals are equal." But to avoid common misunderstandings, I need to be careful to spell out exactly what I mean by this. Obviously nonhuman animals cannot have equal rights to vote and nor should they be held criminally responsible for what they do. That is not the kind of equality I want to extend to nonhuman animals. The fundamental form of equality is equal consideration of interests, and it is this that we should extend beyond the boundaries of our own species. Essentially this means that if an animal feels pain, the pain matters as much as it does when a human feels pain—if the pains hurt just as much. How bad pain and suffering are does not depend on the species of being that experiences it."
January 1, 1970
https://en.wikiquote.org/wiki/Speciesism