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 SingerI believe that nationalism is a very strong force, but there are other forces operating; there are tendencies pushing towards a larger picture, especially in Europe, I think; but I still think nationalism is real.
Peter SingerThe traditional view of the sanctity of human life will collapse under pressure from scientific, technological and demographic developments.
Peter Singer