Consistent with his general ethical theory, Singer holds that the right to life is intrinsically tied to a being's capacity to hold preferences, which in turn is intrinsically tied to a being's capacity to feel pain and pleasure. In his view, the central argument against abortion is equivalent to the following logical syllogism:
First premise: It is wrong to take innocent human life.
Second premise: From conception onwards, the embryo or fetus is innocent, human and alive.
Conclusion: It is wrong to take the life of the embryo or fetus.
In his book Rethinking Life and Death Singer asserts that, if we take the premises at face value, the argument is deductively valid. Singer comments that those who do not generally think abortion is wrong attack the second premise, suggesting that the fetus becomes a "human" or "alive" at some point after conception; however, Singer argues that human development is a gradual process, that it is nearly impossible to mark a particular moment in time as the moment at which human life begins.
Singer's argument for abortion differs from many other proponents of abortion; rather than attacking the second premise of the anti-abortion argument, Singer attacks the first premise, denying that it is wrong to take innocent human life:
[The argument that a fetus is not alive] is a resort to a convenient fiction that turns an evidently living being into one that legally is not alive. Instead of accepting such fictions, we should recognise that the fact that a being is human, and alive, does not in itself tell us whether it is wrong to take that being's life.
Singer states that arguments for or against abortion should be based on utilitarian calculation which weighs the preferences of a mother against the preferences of the fetus. In his view a preference is anything sought to be obtained or avoided; all forms of benefit or harm caused to a being correspond directly with the satisfaction or frustration of one or more of its preferences. Since a capacity to experience the sensations of suffering or satisfaction is a prerequisite to having any preferences at all, and a fetus, at least up to around eighteen weeks, says Singer, has no capacity to suffer or feel satisfaction, it is not possible for such a fetus to hold any preferences at all. In a utilitarian calculation, there is nothing to weigh against a mother's preferences to have an abortion, therefore abortion is morally permissible.
Similar to his argument for abortion, Singer argues that newborns similarly lack the essential characteristics of personhood — "rationality, autonomy, and self-consciousness" — and therefore "killing a newborn baby is never equivalent to killing a person, that is, a being who wants to go on living."
Singer classifies euthanasia as voluntary, involuntary, or non-voluntary. Voluntary euthanasia is that with the consent of the subject.
Singer's book Rethinking Life and Death: The Collapse of Our Traditional Ethics offers further examination of the ethical dilemmas concerning the advances of medicine. He covers the value of human life and quality of life ethics in addition to abortion and other controversial ethical questions.
Singer has experienced the complexities of some of these questions in his own life. His mother had Alzheimer's disease. He said, "I think this has made me see how the issues of someone with these kinds of problems are really very difficult". In an interview with Ronald Bailey, published in December 2000, he explained that his sister shares the responsibility of making decisions about his mother. He did say that, if he were solely responsible, his mother might not continue to live.