In a similar way the discussion of other fundamental questions, such as the real nature of mind, the relation of mind and body in life and death, becomes prejudiced and confused on account of their connection with the deepest-rooted sentiments and longings of the human race. In recent years this has been the case especially in connection with the question of the evolution of mental life from its lower forms in the animals to its higher in man. What ought to be taught and investigated on its own merits as pure scientific theory, as the probable meaning of experienced facts, comes to be a matter of belief and good character, or is considered a sign of courageous independence of spirit and superiority to superstition and traditional prejudice. All of this is quite comprehensible when we consider the enormous practical importance of the questions at issue. Yet such an attitude will scarcely be of much help in finding answers most correct from a purely objective standpoint; it rather discourages the advance of research along definite lines. Nevertheless, as we have stated in the beginning, psychology has now entered upon a positive development. What favorable circumstances have made it possible to overcome, at least in part, the peculiar opposing difficulties? There are many; but in the end they all lead back to one: the rise and progress of natural science since the sixteenth century. However, this has made itself felt in two quite different ways; the force of the first wave was increased to its full magnitude by a closely following second wave. First, natural science served--if we overlook the hasty identification of mind and matter which had its origin in natural science--as a shining and fruitful example to psychology. It suggested conceptions of mental life analogous to those conceptions which had been found to make material processes comprehensible. It led to attempts at employing methods similar to those which had proved valuable in natural science. This influence was especially active in the seventeenth and eighteenth centuries, and lasted into the nineteenth. Later a more direct influence began to make itself felt: an actual invasion by natural science of special provinces of psychology. Natural science, in the course of its further development, was led at many points into investigations which lay as well in the sphere of psychology as in its own prescribed paths. When it attacked them and worked out beautiful solutions for them, psychologists also received a strong impulse not to stand aside, but to take up those problems themselves and pursue them independently for their own quite different purposes. So it was in the nineteenth century, especially in its second half. Let us discuss more in detail a few particular results of this twofold general influence.