Let us now consider the connection of the four principles we have discussed. The second and fourth may be looked upon as special applications of the two fundamental principles of creative resultants and conditioning relations. We see also that they are not only joined very closely together, but that they stand, as absolutely disparate incomparable laws, in contradistinction to those general principles to which all natural phenomena are subjected. A contradiction has very often been thought to exist in the relation between the universal mental and natural laws. And since the natural laws are considered to be the more general and more necessary, these psychical principles have been looked upon as inadmissible generalisations, if they have not been absolutely ignored, which has more often been the case. Now we have seen in our whole discussion, that in reality we cannot move a step in the interpretation of psychical processes from the simplest sense-perceptions and affective combinations to the most complicated mental processes, as shown in society and history, without meeting with these principles always and everywhere. We must of course keep strictly to the maxim of analysing the psychical processes in their own connections and as processes joined together in themselves, as far as they so appear to us. Now the neglect of this maxim has led to the above-mentioned contradiction and to the disregard of these laws. In fact the reverse maxim has been formed, namely, that psychical processes should not be held as decisive for the principles that condition them, but rather that the laws of nature, founded upon external natural phenomena, should also rule our mental life. In this sense the law of the conservation of energy has been considered a fundamental law of all mental development. For this purpose, and also in order to preserve a kind of independence for our mental life, the conception of "psychical energy" has been formed. This, in all the changes it undergoes, is supposed to be subject to the law of the conservation of energy just like mechanical, thermal, electro-magnetic, or any other energy. Since we do not possess a definite unit of measurement for this psychical energy, and since it always occurs between two other physical energies, it was taken for granted that it could be indirectly measured. It could be placed in the middle of a series of transformations that took place according to constant equivalents as a value to be measured indirectly by the physical energy equivalent to it. For example, it could be placed between a given quantity of chemical energy, supplied from outside to the organism, and an equivalent quantity of warmth and mechanical work-energy, which the organism produces. If this were the case we could not reconcile with it a principle such as the one of creative resultants. Such a principle could not be included among the universal psychical laws; it would have to lie outside the general regularity of our psychical life.