The scientist who considers himself absolutely "objective" and believes that he can free himself from the compulsion of the "merely" subjective should try — only in imagination of course — to kill in succession a lettuce, a fly, a frog, a guineapig, a cat, a dog, and finally a chimpanzee. He will then be aware how increasingly difficult murder becomes as the victim's level of organisation rises. The degree of inhibition against killing each one of these beings is a very precise measure for the considerably different values that we cannot help attributing to lower and higher forms of life. To any man who finds it equally easy to chop up a live dog and a live lettuce I would recommend suicide at his earliest convenience!

Ch. XII : On the Virtue of Scientific Humility - On Aggression (1963)