The assumption involved in the former change is that made by comparative or evolutionist psychology, when it attempts to play the part of a theory of knowledge. The development of impressions and ideas is made to pass upwards to more complicated stages, till it reaches the point at which the individual, conceived as determined by external forces and reacting upon them, becomes conscious of itself as a subject of knowledge and source of action. This transition from the category of causality to self-consciousness is, in some systems--that of Mr Spencer, for example--either concealed or held to with no firm grasp. Throughout his objective treatment of psychology, it would seem that Mr Spencer is evolving mind or self-consciousness out of the process in which simple relations of matter and motion form the lowest stage, and reflex action is that which approaches most nearly to having mental characteristics. And, from this objective point of view, he speaks of his philosophy as an interpretation of "the detailed phenomena of life, mind, and society, in terms of matter, motion, and force."[281] But when he discusses the subjective side, he admits that it is entirely unique and _sui generis_,[282] and adopts what is known as the "two aspects" theory--the theory that mind cannot be accounted for as derived from matter, any more than matter can be accounted for as derived from mind, but that they are both phases of one ultimate and unknown reality.[283] This admission involves a practical acknowledgment that it is impossible to arrive at consciousness or at subjectivity by a process of natural development. We must, it affirms, postulate two aspects or phases of existence, or two lines of development, connected probably in their ultimate reality, but, as known to us, distinct from one another, and without mutual influence. [Sidenote: Reference to self-consciousness,] The doctrine that a reference to self-consciousness is implied in experience, may perhaps be made clearer by considering a criticism to which it has recently been subjected by an able psychological writer. Professor W. James writes as follows:--