Sentience is a very subjective and abstract concept. If we want to use sentience as a measure of moral considerations, we must have a unified standard to measure the pain and pleasure of all beings. However, it is hard for humans to learn about how other animals feel.
Humans’ capacity for pleasure and pain for themselves is indisputable because one can directly sense them. However, to know whether other people or animals have a similar ability to experience pain and pleasure requires more solid proofs. How can people know that others have the same sense of pleasure and pain as themselves? And In particular, how can people learn that an animal that is biologically different from ourselves is capable of feeling the same pleasure and pain with humans? People cannot know the ability of other than themselves to perceive pain and pleasure. The conclusion that animals have the same ability to feel pain and pleasure is only derived from humans’ own experiences, which is not an objective fact. If it is not possible to prove that animals have a similar ability to feel pain and pleasure is objective and not derived from human will, it means that Peter Singer’s measure standards lack an objective basis.