Excerpt from Space and Vision: An Attempt to Deduce All Our Knowledge of Space From the Sense of Sight, With a Note on the Association Psychology

Emission and Wave Theories of Light, nor in a thunder storm that we need look for a test of the Single and Double-fluid hypotheses in Electricity. The same observa tion will apply to theories in Psychology; and the writer who confines himself to the explanation of mere generali ties will never establish his theory so long as there is another in the field. I should, therefore, have desired to make my own explanations more special than they are; but finding I could not devote enough time to the subject to enable me to reconsider and rewrite it completely, I have contented myself with making a few corrections and insertions in a manuscript that has been lying by me for some years. The question is one with which Trinity College, Dublin, is peculiarly identified, through Bishop Berkeley and Professor Abbott; and I trust she will not suffer it silently to fall into the hands of inquirers of other nations. Berkeley's theory of vision led directly to his now-celebrated Idealism; and if there is any mode of escaping from that subtle doctrine, I believe it must be discovered in the same field.

There is a current doctrine on this topic which is so closely related to the subject of this Essay, that I may be excused for touching briefly on it here, viz.: That tactual sensations and resistance are the great tests of material existence and reality. I find this doctrine maintained by two thinkers of such Opposite schools as Messrs. Mill and Manse]. It seems to have originated in a confusion between vulgar and the philosophical conception of matter. The vulgar include in their idea cohesion of the particles, and can hardly be persuaded to regard liquids and gases as material, until they see them enclosed in some solid body which gives them a kind of artificial coherence. Philosophers took up the same idea with little examination, and thought the Newtonian theories of gravitation and inertia afforded astrong confirmation of it. But if by matter we mean the Real in Space - that which exists therein and affects our senses - it is now certain that a great part of the material universe consists of imponderable and penetrable fluids, which would probably offer no resistance to the finest sensitive or muscular apparatus. This I take to be the proper philosophical idea of matter, and, in this sense of the term, sight is quite as competent to reveal to us the existence of matter as touch or the motor nerves. In this respect I think Kant, whose tests of material substance are permanence and action, is considerably in advance of more recent writers. Visible unresisting objects are no doubt often regarded as unreal; but this is not the case when they continue to exist permanently, and affect our senses of hearing or smell otherwise smoke would appear as unreal as a ghost. How ever, if anyone desires to confine the term matter to tangible bodies, he is at liberty to do so. I only contend that. The eye reveals to us one spacial reality, namely, the nervous organism connected with it, and that we can thence reach other spacial realities outside us by a legitimate in ference. If so, whether the existence of matter is proved or no, Berkeley's Idealism is refuted.

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