If a being suffers there can be no moral justification for refusing to take that suffering into consideration. No matter what the nature of the being, the principle of equality requires that its suffering be counted equally with the like suffering - insofar as rough comparisons can be made - of any other being. So the limit of sentience is the only defensible boundary of concern for the interests of others. To mark this boundary by some other characteristic like intelligence or rationality would be to mark it in an arbitrary manner. Why not choose some other characteristic, like skin color?
Peter SingerThere has been opposition to experimenting on animals for a long time. This opposition has made little headway because experimenters, backed by commercial firms that profit by supplying laboratory animals and equipment, have been able to convince legislators and the public that opposition comes from uninformed fanatics who consider the interests of animals more important than the interests of human beings.
Peter SingerWere we incapable of empathy โ of putting ourselves in the position of others and seeing that their suffering is like our own โ then ethical reasoning would lead nowhere. If emotion without reason is blind, then reason without emotion is impotent.
Peter Singer