The fact of sexual need in man and animal is expressed in biology by the assumption of a "sexual impulse." This impulse is made analogous to the impulse of taking nourishment, and to hunger. The sexual expression corresponding to hunger not being found colloquilly, science uses the expression "libido."

Popular conception makes definite assumptions concerning the nature and qualities of this sexual impulse. It is supposed to be absent during childhood and to commence about the time of and in connection with the maturing process of puberty; it is supposed that it manifests itself in irresistible attractions exerted by one sex upon the other, and that its aim is sexual union or at least such actions as would lead to union. But we have every reason to see in these assumptions a very untrustworthy picture of reality. On closer examination they are found to abound in errors, inaccuracies and hasty conclusions.