Becoming a Vegan

How far down the evolutionary scale shall we go? Shall we eat fish? What about shrimps? Oysters? To answer these questions we must bear in mind the central principle on which our concern for other beings is based...the only legitimate boundary to our concern for the interests of other beings is the point at which it is no longer accurate to say that the other being has interests. To have interests, in a strict, nonmetaphorical sense, a being must be capable of suffering or experiencing pleasure. If a being suffers, there can be no moral justification for disregarding that suffering, or for refusing to count it equally with the like suffering of any other being. But the converse of this is also true. If a being is not capable of suffering, or of enjoyment, there is nothing to take into account.

Peter Singer
Becoming a Vegan