I think you will already understand the position we have reached with these considerations. We are about to follow up the energic conception by putting the energic mode of action in place of the purely formal functioning. Just as reciprocal actions, well known in the old natural science, have been replaced by the law of the conservation of energy, so here too, in the sphere of psychology, we seek to replace the reciprocal activities of coordinated psychical faculties by energy, conceived as one and homogeneous. Thus we must bow to the criticism which reproaches the psychoanalytic school for working with a mystical conception of libido. I have to dispel this illusion that the whole psychoanalytic school possesses a clearly conceived and obvious conception of libido. I maintain that the conception of libido with which we are working is not only not concrete or known, but is an unknown _X_, a conceptual image, a token, and no more real than the energy in the conceptual world of the physicist. In this wise only can we escape those arbitrary transgressions of the proper boundaries, which are always made when we want to reduce coördinated forces to one another. Certain analogies of the action of heat with the action of light are not to be explained by saying that this tertium comparationis proves that the undulations of heat are the same as the undulations of light; the conceptual image of energy is the real point of comparison. If we regard libido in this way we endeavor to simulate the progress which has already been made in physics. The economy of thought which physics has already obtained we strive after in our libido theory. We conceive libido now simply as energy, so that we are in the position to figure the manifold processes as forms of energy. Thus, we replace the old reciprocal action by relations of absolute equivalence. We shall not be astonished if we are met with the cry of vitalism. But we are as far removed from any belief in a specific vital power, as from any other metaphysical assertion. We term libido that energy which manifests itself by vital processes, which is subjectively perceived as aspiration, longing and striving. We see in the diversity of natural phenomena the desire, the libido, in the most diverse applications and forms. In early childhood we find libido at first wholly in the form of the instinct of nutrition, providing for the development of the body. As the body develops, there open up, successively, new spheres of influence for the libido. The last, and, from its functional significance, most overpowering sphere of influence, is sexuality, which at first seems very closely connected with the function of nutrition. With that you may compare the well-known influence on propagation of the conditions of nutrition in the lower animals and plants.