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.

I am not saying that factory farming is the same as the Holocaust or the slave trade, but it's clear that there is an immense amount of suffering in it, and just as we think that the Nazis were wrong to ignore the suffering of their victims, so we are wrong to ignore the sufferings of our victims.

More often there's a compromise between ethics and expediency.

Killing a defective infant is not morally equivalent to killing a person. Sometimes it is not wrong at all.

If we're going to live an ethical life, it's not enough just to follow the thou-shalt-nots. … If we have enough, we have to share some of that with people who have so little.

The statement "I am in pain" may be one piece of evidence for the conclusion that the speaker is in pain, but it is not the only possible evidence, and since people sometimes tell lies, not even the best possible evidence. Even if there were stronger grounds for refusing to attribute pain to those who do not have language, the consequences of this refusal might lead us to reject the conclusion. Human infants and young children are unable to use language. Are we to deny that a year-old child can suffer?

When fish experience something that would cause other animals physical pain, they behave in ways suggestive of pain, and the change in behaviour may last several hours.