[W]e cannot concentrate only on benefactors to humans. Perhaps Peter Singer, the most influential person in promoting the welfare and rights of animals, will ultimately have contributed more to the development of the universe than benefactors merely of humans. Perhaps Singer’s book Animal Liberation will (over the centuries) have increased the happiness, health, and lives of animals to such an extent that it adds up to a greater amount of goodness than the human development that will be the total consequence of (say) Gandhi’s actions.
Quentin Smith, Ethical and Religious Thought in Analytic Philosophy of Language, New Haven, 1997, p. 217
I regard Peter as one of the great moralists, because I suspect that more than anyone he has helped to change the attitudes of very many people to the sufferings of animals. Peter is a utilitarian in normative ethics, and a humane attitude to animals is a natural corollary of utilitarianism. Utilitarian concern for animals goes back to Bentham, who, presumably alluding to the Kantians, said that the question was not whether animals can reason, but whether they can suffer.
J. J. C. Smart, ‘Reply to Singer’, in Philip Pettit, Richard Sylvan and Jean Norman (eds.), Metaphysics and Morality: Essays in Honour of J. J. C. Smart, Oxford, 1987, p. 192