“This view of a living nature where man is nothing is both odd and sad. Here, in a fertile land, in an eternal greenness, you search in vain for traces of man; you feel you are carried into a different world from the one you were born into.”

“Die gefährlichste Weltanschauung ist die Weltanschauung derer, die die Welt nie angeschaut haben. (The most dangerous worldview is the worldview of those who have not viewed the world)”

“With most animals, as with man, the alertness of the senses diminishes after years of work, after domestic habits and progress of culture.”

“There are three stages of scientific discovery: first people deny it is true; then they deny it is important; finally they credit the wrong person.”

“It is an absolutely vain endeavor to attempt to reconstruct or even comprehend the nature of a human being by simply knowing the forces which have acted upon him. However deeply we should like to penetrate, however close we seem to be drawing to truth, one unknown quantity eludes us: man's primordial energy, his original self, that personality which was given him with the gift of life itself. On it rests man's true freedom; it alone determines his real character.”

“Human nature must be something which always remains one and the same, but which may be carried out in manifold ways.”

“I am more and more convinced that our happiness or unhappiness depends more on the way we meet the events of life than on the nature of those events themselves.”