Unfortunately for ethical egoism, the claim that we will all be better off if every one of us does what is in his or her own interest is incorrect. This is shown by what are known as "prisoner's dilemma" situations, which are playing an increasingly important role in discussions of ethical theory... At least on the collective level, therefore, egoism is self-defeating - a conclusion well brought out by Parfit in his aforementioned Reasons and Persons.
Peter SingerAccording to classical utilitarianism, the only intrinsic good is happiness; the only intrinsic bad is pain. That implies no intrinsic value in preserving nature, that preserving an endangered plant is valuable only if it benefits humans or other animals. Intuitively, that seems wrong but perhaps I shouldn't trust my intuition here.
Peter SingerWhen you look at food as an ethical issue in the Christian tradition, you don't find very much about it. You don't find, as you do in the Jewish or Islamic or Hindu traditions, a lot of restrictions saying you can eat this but you can't eat that. But what you do find is the idea that gluttony is a sin and that it's something that we ought to be ashamed of.
Peter Singer