Why is it thought morally right to kill an animal to end their suffering yet morally wrong to kill a human to end their suffering?

There's clearly an enormous amount that could be said about this, but here are a few thoughts.

Suppose that some person is suffering, and to avoid certain complications, suppose that there's no "cure" for their pain. Now suppose that the person actually wants us to take his life. (Imagine that he isn't in a position to do it himself.) Then it's not just obvious that it is wrong, all things considered, to kill him. That's why there's a serious debate about euthanasia.

That said, there are important differences between typical human beings and most other animals: humans don't just have immediate desires and aversions; humans have self-concepts which include plans, desires and values that bear on their own futures. Most animals, or so we believe, don't have any such things. We normally think that people's views about their own futures count -- that it's wrong simply to ignore them. In particular, if someone is suffering but doesn't want to die, we think that carries tremendous weight. Most animals, or so we think, don't have the capacity for the relevant thoughts. They don't have a conception of themselves as being who have potential futures about which they have plans, wishes and desires.

That's at best a contingent fact, and it may not be true of all animals. If a creature has a conception of its own future and has desires about the course of that future, then that makes a moral difference -- or so we normally think. And so it may well be that even in the case of some non-human animals, killing a creature in order to end its suffering is wrong.

None of this is meant to address all the issues that your question raises. Other panelists may well have things to say on matters that I haven't even raised. But the normal human capacity to be able to think about one's future is surely relevant to sorting all this out.

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