"We cannot directly experience anyone else's pain, whether that "anyone"is our best friend or a stray dog. Pain is a state of consciousness, a "mental event, "and as such it can never be observed. Behaviour like writhing, screaming, or drawing one's hand away from the lighted cigarette is not pain itself; nor are the recordings a neurologist might make of activity within the brain observations of pain itself. Pain is something that we feel, and we can only infer that others are feeling it from various external indications . . . "( Singer, 1990 ).
Many environmentalists like Aldo Leopold and Holmes Rolston III criticize as unecological the emphasis those animal liberationists like Peter Singer place on preventing animal suffering. Their argument holds that animal liberationists must regard predation as bad by extending the role of moral consideration to include things that do not feel pain or pleasure. The view that Leopold and Rolston hold maintain that like the natural system of the animal world ( humans and non-human animals ), the eco systems of the land (including soil, plants, and all animals) is highly debatable and should be taken into moral consideration. This paper disputes the latter view by arguing that the capacity for suffering and enjoying things is a prerequisite for having interests at all.
As Holmes Rolston III argues, "Singer has himself proved blind to the still larger effort in environmental ethics to value life in all its ranges and levels, indeed to care for biosphere Earth . . . His victory is mainly for vertebrates, who form only 4 percent of living things by species and only a tiny fraction of a per cent by numbers of individuals"( Rolston, 1990 ). It is reasonable to suppose all vertebrate animals to be capable of feeling pain. With invertebrates the matter becomes unclear but some invertebrates also seem likely to experience pain. As Singer says, "In each case we must look at the nature of the creature and its nervous system.