Published in 1975, Animal Liberation has been cited as a formative influence on leaders of the modern animal liberation movement. The central argument of the book is an expansion of the utilitarian idea that “the greatest good of the greatest number” is the only measure of good or ethical behaviour. Singer argues that there is no reason not to apply this to other animals. He introduced and popularized the term “speciesism”, which was originally coined by Richard D. Ryder, to describe the practice of privileging humans over other animals.

Singer’s most comprehensive work, Practical Ethics (1979), analyzes in detail why and how living beings’ interests should be weighed. His principle of equal consideration of interests does not dictate equal treatment of all those with interests, since different interests warrant different treatment.  Ethical conduct is justifiable by reasons that go beyond prudence to “something bigger than the individual,” addressing a larger audience. Singer thinks this going-beyond identifies moral reasons as “somehow universal”, specifically in the injunction to ‘love thy neighbor as thyself’, interpreted by him as demanding that one give the same weight to the interests of others as one gives to one’s own interests. This universalising step, which Singer traces from Kant to Hare, is crucial and sets him apart from those moral theorists from Hobbes to David Gauthier, who tie morality to prudence. Universalisation leads directly to utilitarianism, Singer argues, on the strength of the thought that one’s own interests cannot count for more than the interests of others. Taking these into account, one must weigh them up and adopt the course of action that is most likely to maximise the interests of those affected; utilitarianism has been arrived at. Singer’s universalising step applies to interests without reference to who has them, whereas a Kantian’s applies to the judgments of rational agents (in Kant’s kingdom of ends, or Rawls’s Original Position, etc.). Singer regards Kantian universalization as unjust to animals.  As for the Hobbesians, Singer attempts a response in the final chapter of Practical Ethics, arguing that self-interested reasons support adoption of the moral point of view, such as ‘the paradox of hedonism’, which counsels that happiness is best found by not looking for it, and the need most people feel to relate to something larger than their own concerns.