Chemical Theories.—­The truth remains, however, that we are unable to give any information about the nature of the sexual excitement for the reason that we do not know with what organ or organs sexuality is connected, since we have seen that the sexual glands have been overestimated in this significance.  Since surprising discoveries have taught us the important role of the thyroid gland in sexuality, we may assume that the knowledge of the essential factors of sexuality are still withheld from us.  One who feels the need of filling up the large gap in our knowledge with a preliminary assumption may formulate for himself the following theory based on the active substances found in the thyroid.  Through the appropriate excitement of erogenous zones, as well as through other conditions under which sexual excitement originates, a material which is universally distributed in the organism becomes disintegrated, the decomposing products of which supply a specific stimulus to the organs of reproduction or to the spinal center connected with them.  Such a transformation of a toxic stimulus in a particular organic stimulus we are already familiar with from other toxic products introduced into the body from without.  To treat, if only hypothetically, the complexities of the pure toxic and the physiologic stimulations which result in the sexual processes is not now our appropriate task.  To be sure, I attach no value to this special assumption and I shall be quite ready to give it up in favor of another, provided its original character, the emphasis on the sexual chemism, were preserved.  For this apparently arbitrary statement is supported by a fact which, though little heeded, is most noteworthy.  The neuroses which can be traced only to disturbances of the sexual life show the greatest clinical resemblance to the phenomena of intoxication and abstinence which result from the habitual introduction of pleasure-producing poisonous substances (alkaloids.)

THE THEORY OF THE LIBIDO

These assumptions concerning the chemical basis of the sexual excitement are in full accord with the auxiliary conception which we formed for the purpose of mastering the psychic manifestations of the sexual life.  We have determined the concept of libido as that of a force of variable quantity which has the capacity of measuring processes and transformations in the spheres of sexual excitement.  This libido we distinguished from the energy which is to be generally adjudged to the psychic processes with reference to its special origin and thus we attribute to it also a qualitative character.  In separating libidinous from other psychic energy we give expression to the assumption that the sexual processes of the organism are differentiated from the nutritional processes through a special chemism.  The analyses of perversions and psychoneuroses have taught us that this sexual excitement is furnished not only from the so-called sexual parts alone but from all organs of the body.  We thus formulate for ourselves the concept of a libido-quantum whose psychic representative we designate as the ego-libido; the production, increase, distribution and displacement of this ego-libido will offer the possible explanation for the observed psycho-sexual phenomena.