There should be no difficulty in deciding what the facts permit us to postulate. The subconscious is a theory based upon observed facts and formulated to explain those facts. There are many precise phenomena of different kinds which can only be explained as due to explicitly subconscious processes, that is, processes which do not appear in the content of consciousness; just as the phenomena manifested by radium can only be explained by emanations (or rays) which themselves are not visible and cannot be made the object of conscious experience. In each case it is the manifestations of such processes of which we become aware. Subconscious processes and radio-activity stand on precisely the same basis so far as the determination of their actuality is concerned. (The latter have the advantage, of course, in that being physical they are subject to quantitative measurement.) Such being the case it ought to be possible to construct the theory of the subconscious by inductive methods on the basis of facts of observation just as any theory of the physical sciences is constructed. This task I have set before myself as well as that of giving precision to our conception of the theory and taking it out of the domain of philosophical concepts. With this purpose in view I have endeavored to apply the method of science and construct the theory by induction from the data of observation and experiment. I dare say this has been a somewhat ambitious and some will say, perhaps, overbold undertaking. Undoubtedly, too, this attitude toward this and other individual problems has not been always consistently maintained, nor perhaps is it completely possible in the present state of the science. Our formulations should be as precise as possible and facts and concepts of a different order should not be included in one and the same formula. I have, accordingly, divided the subconscious into two classes, namely (1) the _unconscious_, or neural dispositions and processes, and (2) the _coconscious_, or actual subconscious ideas which do not enter the content of conscious awareness. An unconscious process and a coconscious process are both therefore _subconscious_ processes but particular types thereof—the one being purely neural or physical and the other psychological or ideational. The soundness of the conclusions reached in this work I leave to the judgment of my critics, of whom I doubt not I shall have many. I do not hesitate to say, however, that it is only by practical familiarity with the phenomena of mental pathology and artificially induced phenomena (such as those of hypnosis, suggestion, etc.), requiring a long training in this field of research (as in other scientific fields), that we can correctly estimate the value of data and the conclusions drawn therefrom; and even then many of our conclusions can be regarded as only provisional.