Now it appears to the uninitiated as if the spiritual world of science were in every respect different from the world of objective realities around us. While in the world of bodily realities the struggle for existence is fought eternal peace is supposed to reign in the sacred halls of intellectual aspirations. Says the German poet: “_Härt in dem Raume stossen sich die Körper,_ _Leicht bei einander wohnen die Gedanken._” This is true only in a very limited sense. Ideas are the most intolerant beings imaginable. The struggle for existence is raging as fiercely in the intellectual realm as in the world of realities, and there also the law that the fittest will survive holds good. Far be it from us to denounce this state of general warfare, for although it is hard on those who succumb, it is the means by which evolution becomes possible; and evolution in the domain of science means a nearer approach to truth. If in the evolution of thought two neighboring sciences have developed so far as to meet, a struggle will ensue. The ideas on the two sides will have to fight before they coalesce. It is natural that different scientists look at things from different standpoints. They have developed a terminology which exactly suits their purpose and thus the representatives of the different sciences are often like people of a different nationality. They do not understand each other because they speak different languages. Moreover they have not unfrequently a different religion; that means, their ideas about truth and the test of truth appear to be different and sometimes they regard one another as no better than heathens. The battle is unavoidable, and considering all in all, the battle is desirable, it should not be avoided. The fittest to survive being the truest, the whole progress of science through the struggle for existence among ideas consists in the approach to truth. It may be objected that there are peacemakers who will reconcile the contending parties. True. And it is further true that the aim of every war is peace. But a peacemaker can be successful only if his mind is broad enough to let the whole battle be fought out within himself. The battle itself is and will remain unavoidable. Idea stands against idea, and the mental process of reflection is nothing but a struggle of conflicting ideas which takes place in one and the same mind. The aim of all reflection is the settlement of the conflict, so that all ideas will agree. The two parties disappear in one; errors are given up, and that which is consistent only will remain. In other words Dualism makes room for Monism.