In Animal Liberation, Singer rests his case against animal research, agriculture, hunting, etc. on two generic argument strategies, both relying on the concept of speciesism.
The Argument From Marginal Cases: The label for this form of argument may be unfortunate, but it is widely used in the contemporary literature on animal rights. By "marginal cases" are meant human beings who lack many, most, or all of the intellectual traits distinctive of normal adult humans, e.g. severely retarded persons, fetuses and neonates, and the irreversibly comatose. An argument from marginal cases proceeds by
"[I]f we consider it wrong to inflict pain on a baby for no good reason then we must, unless we are speciesists, consider it equally wrong to inflict the same amount of pain on a horse for no good reason" (p. 15).
The Generalization Argument: This kind of argument works the other way around.
Here is an example of this type of argument from Singer:
"[I]f we use [the argument that certain experiments would cause less pain if performed on non-human animals than if performed on normal human beings] to justify experiments on nonhuman animals we have to ask ourselves whether we are also prepared to allow experiments on human infants and retarded adults . . . " (p. 16)