Interpretations of the psychical such as these have given rise to four peculiar “isms” of an epistemological nature, _i.e._, related to a theory of knowledge. Not infrequently they are the historical antecedents which result in the naturalistic theory of the psychical. These are nominalism and sensualism, empiricism and a-posteriorism, which, setting themselves against epistemological rationalism, assail the dignity, the independence, and the autonomy of the thinking mind. They are so necessarily and closely associated with naturalism that their fate is intimately bound up with its fate, and they are corroborated or refuted with it. And it would be possible to conduct the whole discussion with which we are concerned purely with reference to these four “isms.” The strife really begins in their camp. The soul is a _tabula rasa_, all four maintain, a white paper on which, to begin with, nothing is inscribed. It brings with it neither innate knowledge nor commands. What it possesses in the way of percepts, concepts, opinions, convictions, principles of action, rules of conduct, are inscribed upon it through experience (empiricism). That is, not antecedent to, but subsequent to experience (a posteriori). But experience can only be gained through the senses. Only thus does reality penetrate into and stamp itself upon us. “What was not first in the senses (sensus) cannot be in the intelligence.” What the senses convey to us alone builds up our mental content, from mere sensory perceptions upwards to the most abstract ideas from the simplest psychical elements up to the most complex ideas, concepts, and conclusions, to the most varied imaginative constructions. And in the development of the mental content the “soul” itself is merely the stage upon which all that is acquired through the senses crowds, and jostles, and unites to form images, perceptions, and precepts. But it is itself purely passive, and it becomes what happens to it. Therefore it is not really spirit at all, for spirit implies spontaneity, activity, and autonomy. Philosophy and the mental sciences have always had to carry on the strife with these four opponents. And it is in the teacup of logic and epistemology that the storm in regard to theories of the universe has arisen. It is there, and not in the domain of neurology, or zoology, that the real battlefield lies, upon which the controversy must be fought out to the end. What follows is only a sort of skirmish about the outposts.