Religious Vegetarianism, like the Seventh-day Adventist Church and the Utilitarian Philosophy are the two pillars of the philosophy for Veganism. Utilitarianism is a version of consequentialism, which states that the consequences of any action are the only standard of right and wrong, good and bad, pleasure and pain etc. According to Peter Singer, sentient beings have to be included when acting.
The central argument in Peter Singer's book Animal Liberation, is an expansion of the utilitarian concept that "the greatest good of the greatest number" is the only measure of good or ethical behaviour. Singer believes that there is no reason not to apply this principle to other animals, arguing that the boundary between human and "animal" is completely arbitrary.
There is a lot of criticism about Utilitarianism. It is e.g. impossible to precisely predict the consequences and they are inherently unknowable. Another idea of this normative Philosophy is the duty of the decision maker (Agent) to put his own interest on side. The well-being of strangers counts just as much as that of friends, family or self. And this idea is contrary to other Philosophies. E.g. Ethical egoism is the normative ethical position that moral agents ought to act in their own self-interest.
A thought experiment - the Trolley Problem
The trolley problem is a thought experiment as an ethical dilemma of whether to sacrifice one person to save a larger number of persons.
In the following I will add another version to this thought experiment, by taking Peter Singer's words and take his idea to extent this thought experiment with sentient beings.