Wolfgang Kohler quotes:

It has sometimes been said that we find nowhere in nature an analogue of the difference between happens and is, on the one hand, and ought, on the other hand.

It would be interesting to inquire how many times essential advances in science have first been made possible by the fact that the boundaries of special disciplines were not respected... Trespassing is one of the most successful techniques in science.

Psychology is a very unsatisfactory science.

If we wish to imitate the physical sciences, we must not imitate them in their contemporary, most developed form; we must imitate them in their historical youth, when their state of development was comparable to our own at the present time. Otherwise we should behave like boys who try to copy the imposing manners of full-grown men without understanding their raison d' être, also without seeing that in development one cannot jump over intermediate and preliminary phases.

It is hardly an exaggeration to say that a chimpanzee kept in solitude is not a real chimpanzee at all.