"Nothing can better express the feelings of the scientist towards the great unity of the laws of nature than in Immanuel Kant's words: "Two things fill the mind with ever new and increasing awe: the stars above me and the moral law within me."… Would he, who did not yet know of the evolution of the world of organisms, be shocked that we consider the moral law within us not as something given, a priori, but as something which has arisen by natural evolution, just like the laws of the heavens?"

“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!”

“All the advantages that man has gained from his ever-deepening understanding of the natural world that surrounds him, his technological, chemical and medical progress, all of which should seem to alleviate human suffering... tends instead to favor humanity's destruction.”

“We are the highest achievement reached so far by the great constructors of evolution. We are their "latest" but certainly not their last word. The scientist must not regard anything as absolute, not even the laws of pure reason. He must remain aware of the great fact, discovered by Heraclitus, that nothing whatever really remains the same even for one moment, but that everything is perpetually changing. To regard man, the most ephemeral and rapidly evolving of all species, as the final and unsurpassable achievement of creation, especially at his present-day particularly dangerous and disagreeable stage of development, is certainly the most arrogant and dangerous of all untenable doctrines. If I thought of man as the final image of God, I should not know what to think of God. But when I consider that our ancestors, at a time fairly recent in relation to the earth's history, were perfectly ordinary apes, closely related to chimpanzees, I see a glimmer of hope. It does not require very great optimism to assume that from us human beings something better and higher may evolve. Far from seeing in man the irrevocable and unsurpassable image of God, I assert – more modestly and, I believe, in greater awe of the Creation and its infinite possibilities – that the long-sought missing link between animals and the really humane being is ourselves!”

“Let us imagine that an absolutely unbiased investigator on another planet, perhaps on Mars, is examining human behavior on earth, with the aid of a telescope whose magnification is too small to enable him to discern individuals and follow their separate behavior, but large enough for him to observe occurrences such as migrations of peoples, wars, and similar great historical events. He would never gain the impression that human behavior was dictated by intelligence, still less by responsible morality.”

“The competition between human beings destroys with cold and diabolic brutality.... Under the pressure of this competitive fury we have not only forgotten what is useful to humanity as a whole, but even that which is good and advantageous to the individual.... One asks, which is more damaging to modern humanity: the thirst for money or consuming haste... in either case, fear plays a very important role: the fear of being overtaken by one's competitors, the fear of becoming poor, the fear of making wrong decisions or the fear of not being up to snuff.”

“A chapter from On Aggression”