Carrying a little further this simile, will, I think, show where lies the insuperable difficulty felt by Mr. Sidgwick. Taking _x_ and _y_ as the subjective and objective activities, unknown in their natures and known only as phenomenally manifested; and recognizing the fact that every state of consciousness implies, immediately or remotely, the action of object on subject or subject on object, or both; we may say that every state of consciousness will be symbolized by some modification of xy—the phenomenally-known product of the two unknown factors. In other words, _xy′_, _x′y_, _x′y′_, _x″y′_, _x′y″_, &c., &c., will represent all perceptions and thoughts. Suppose, now, that these are thoughts about the object; composing some hypothesis respecting its characters as analyzed by physicists. Clearly, all such thoughts, be they about shapes, resistances, momenta, molecules, molecular motions, or what not, will contain forms of the subjective activity _x_. Now let the thoughts {248} be concerning mental processes. It must similarly happen that some mode of the unknown objective activity _y_, will be in every case a component. Now suppose that the problem is the genesis of mental phenomena; and that, in the course of the inquiry, bodily organization and the functions of the nervous system are brought into the explanation. It will happen, as before, that these, considered as objective, have to be described and thought about in modes of _xy_. And when by the actions of such a nervous system, conceived objectively in modes of _xy_, and acted upon by physical forces which are conceived in other modes of _xy_, we endeavour to explain the genesis of sensations, perceptions, and ideas, which we can think of only in other modes of _xy,_ we find that all our factors, and therefore all our interpretations, contain the two unknown terms, and that no interpretation is imaginable that will not contain the two unknown terms.