What exactly is the minority moral position for which members of a gathering such as this are likely to be propagandists? It will certainly differ slightly from person to person. There is, however, one rather minimal principle to which I trust that all here would assent, namely that the suffering or frustration of an animal of another species is an evil of the same general sort as is the suffering or frustration of a human being, and that we humans have the same general sort of moral obligation to refrain from causing, and to try actively to prevent, such evil when it concerns animals as when it concerns human beings.

p. 89 - The Animal Welfare Movement and the Foundations of Ethics