Peter Singer said this quote

In an earlier stage of our development most human groups held to a tribal ethic. Members of the tribe were protected, but people of other tribes could be robbed or killed as one pleased. Gradually the circle of protection expanded, but as recently as 150 years ago we did not include blacks. So African human beings could be captured, shipped to America, and sold. In Australia white settlers regarded Aborigines as a pest and hunted them down, much as kangaroos are hunted down today. Just as we have progressed beyond the blatantly racist ethic of the era of slavery and colonialism, so we must now progress beyond the speciesist ethic of the era of factory farming, of the use of animals as mere research tools, of whaling, seal hunting, kangaroo slaughter, and the destruction of wilderness. We must take the final step in expanding the circle of ethics. -

~ Peter Singer

What one generation finds ridiculous, the next accepts; and the third shudders when it looks back on what the first did.

~ Peter Singer

Philosophy ought to question the basic assumptions of the age. Thinking through, critically and carefully, what most of us take for granted is, I believe, the chief task of philosophy, and the task that makes philosophy a worthwhile activity.

~ Peter Singer

To protest about bullfighting in Spain, the eating of dogs in South Korea, or the slaughter of baby seals in Canada while continuing to eat eggs from hens who have spent their lives crammed into cages, or veal from calves who have been deprived of their mothers, their proper diet, and the freedom to lie down with their legs extended, is like denouncing apartheid in South Africa while asking your neighbors not to sell their houses to blacks.

~ Peter Singer