This theory does not, in itself, contain any statement of the mode in which we perceive the material world, whether directly by presentation, or indirectly by representative images; and perhaps it might, without any great violence, be adapted to more than one of the current hypotheses on this point. But that to which it most easily adjusts itself is that maintained by Hamilton himself under the name of _Natural Realism_. To speak of perception as a _relation_ between mind and matter, naturally implies the presence of both correlatives; though each may be modified by its contact with the other. The acid may act on the alkali, and the alkali on the acid, in forming the neutral salt; but each of the ingredients is as truly present as the other, though each enters into the compound in a modified form. And this is equally the case in perception, even if we suppose various media to intervene between the ultimate object and the perceiving mind,--such, _e.g._, as the rays of light and the sensitive organism in vision,--so long as these media are material, like the ultimate object itself. Whether the object, properly so called, in vision, be the rays of light in contact with the organ, or the body emitting or reflecting those rays, is indifferent to the present question, so long as a material object of some kind or other is supposed to be perceived, and not merely an inmaterial representation of such an object. To speak of our perceptions as mere modifications of mind produced by an unknown cause, would be like maintaining that the acid is modified by the influence of the alkali without entering into combination with it. Such a view might perhaps be tolerated, in connection with the theory of relativity, by an indulgent interpretation of language, but it is certainly not that which the language of the theory most naturally suggests. All this Mr. Mill entirely misapprehends. He quotes a passage from Hamilton's Lectures, in which the above theory of Relativity is clearly stated as the mean between the extremes of Idealism and Materialism, and then proceeds to comment as follows:--