Here the problem of the ultimate nature of matter becomes a question of philosophy and metaphysics. It is a field of research by itself. The chemist never confuses that problem with the specific problems of his particular science. These deal with empirical atoms and molecules as he finds them. No chemist would undertake to give the chemical formula of the union of sulphuric acid and zinc by a formula which expressed the ultimate nature of atoms or negative electricity. If he did so he would confuse his problems. And so I think we confuse our problems when we attempt to explain empirical psychological phenomena in philosophical or ultimate terms. We must treat our psychological elements--ideas, wishes, emotions, etc,--as the chemist treats atoms and molecules. But, just as the latter may take up ultimate problems as a special field of investigation so may we do, if we like, but we must not treat them as psychological problems. This confusion of problems is, I think, the fundamental error of Jung and others in treating of the libido when he and they attempt to explain specific phenomena as empirically observed. Jung undertakes to resolve libido into the energy of the universe. Of course this is possible. All forces can be ultimately so resolved, including the forces of mind and body. Emotions such as anger and fear are forces and each of these forces, with great probability, can be reduced in the ultimate analysis to a form of energy. But this is not to admit that we are justified in explaining specific concrete psychological phenomena, with which we are dealing, in philosophical terms. We must explain them in terms of the phenomena themselves. As a monist and pan-psychist, for example, I may believe that conscious processes can be reduced to, or be identified with the ultimate nature of matter, the thing-in-itself. And conversely atoms and electrons may be reduced to a force which may be identified with psychic force, but I would not attempt to explain psychological behaviour in terms of such a philosophical concept but only through phenomenal psychological forces, let us say, wishes. In other words, I would not undertake to introduce pan-psychism into the problem at all as an explanation of a particular phobia. I think, therefore, that when Jung and others attempt to explain phobias and other psychological phenomena through a philosophical concept of the libido as analyzed into an elan vitale or the energy of the universe, they not only confuse their problems but introduce such a mixing up of terms that the resulting explanation becomes little more than nonsense. The libido, whatever it may be, must be treated as a psychophysiological force just like any of the other emotions. Otherwise psychology ceases to be a science.