Before we attempt to answer this question we must note one restriction, to which even in the natural sciences the requirement of universal validity for the leading principles is subject, and which, we may be sure, will be even more prominent in mental science, because of the extraordinarily complex nature of the phenomena. This restriction consists in the fact that the validity of each fundamental principle is subject to certain hypotheses, so that, where these are no longer fulfilled, the principles themselves become doubtful or untenable. Thus the law of the conservation of energy is only valid as long as the measured units of energy belong to a closed or finite material system. It loses its validity if the system is of infinite extent, or if, though finite, it can be acted upon by any external forces. A restriction analogous to this last one will have to be employed in regard to the psychical laws obtained by generalisation from the individual psychological regularities. Of course we must take into account the conditions arising out of the peculiarity of mental phenomena. These psychical laws, by virtue of the subjection of psychical phenomena to the interconnection of consciousness, can only be valid within the limits within which such an interconnection of psychical processes takes place. We shall, for example, try to obtain a fundamental principle which controls the formation of complex psychical processes out of their elements. But it would have no sense to set up such a law for absolutely disparate processes that do not stand in any relation in the single consciousness. It may be that, because of this, the limits of validity for psychological principles are much narrower than those for general natural laws. This is connected with the fact that psychology has to do with inner and not with outer relations. And also we must not forget that this limitation can be compensated for by the character of the psychical laws themselves. And, in fact, the discussion of the first and most general of these laws will show us that this hypothesis proves correct.