_3rd. Psycho-Physical Parallelism._--This is the doctrine maintained by Muensterberg and others. It contends that brain-changes and states of consciousness are merely coincidental in point of time, and do not ever influence each other. Their relation is that of mere coincidence or concomitance, and not causation. The two flow along, side by side, without in any way interfering with one another. As regards this doctrine, it need only be pointed out that, were it true, mind and body could never influence one another, since they are not causally connected. Yet, if there be no connection, how is it that they correspond so exactly?--for, as James said, "It is quite inconceivable that consciousness should have _nothing to do_ with a business which it so faithfully attends." _4th. Phenomenalistic Parallelism._--This is the theory maintained by Kant, Spinoza, and others. It maintains that both brain and consciousness (or mind and body) are but two different expressions of one underlying reality--just as the convex and concave surfaces of a sphere are but two expressions of an underlying reality. As to the nature of this reality, Kant and Herbert Spencer were content to call it X or the unknown, while Spinoza maintained that it was God. Analogies which are held to support this doctrine are, however, extremely defective; but the subject is too lengthy and technical to elucidate in detail here. _5th. Psychical Monism._--This doctrine contends that consciousness is the only reality--the material world being external appearance only. Thoughts are causally connected, but not physical events. (The doctrine is thus the exact inverse of epiphenomenalism.) In refutation of this theory, it may be pointed out that, if brain-changes are thus caused by, or are the outer expressions of, thought--why not muscular changes, and in fact all physical phenomena throughout the world everywhere? For we cannot rationally draw the line of distinction here. Such is the logical outcome of the theory--and has, in fact, been accepted in this form by Fechner and others. While many philosophers are inclined to accept this view, it may be stated that the physical scientists are, naturally, repelled by it, and so is common sense! _6th. Solipsism._--The contention of this theory is that nothing exists save states of consciousness in the individual. Neither the material world nor other minds exist, save in the mind of the individual. This doctrine is so opposed to common sense and daily experience that it is unnecessary to dwell upon it. _7th. Inter-Actionism (Animism)._--Here we have the world-old notion of soul and body existing as separate entities, influencing each other. Mind is here supposed to influence matter, and utilize it for the purposes of its manifestation.