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98 attracted the notice of so many profound thinkers, had it not been for a peculiar difficulty connected with our notions of colour, of which I do not know any one English philosopher who seems to have been sufficiently aware. That this quality belongs to the same class with sounds, smells, tastes, heat and cold, is equally admitted by the partizans of Descartes and of Locke; and must, indeed, appear an indisputable fact to all who are capable of reflecting accurately on the subject. But still, between colour and the other qualities now mentioned, a very important distinction must be allowed to exist. In the case of smells, tastes, sounds, heat and cold, every person must immediately perceive, that his senses give him only a relative idea of the external quality; in other words, that they only convey to him the knowledge of the existence of certain properties or powers in external objects, which fit them to produce certain sensations in his mind; and accordingly, nobody ever hesitated a moment about the truth of this part of the Cartesian philosophy, in so far as these qualities alone are concerned. But, in the application of the same doctrine to colour, I have conversed with many, with whom I found it quite in vain to argue; and this, not from any defect in their reasoning powers, but from their incapacity to reflect steadily on the subjects of their consciousness; or rather, perhaps, from their incapacity to separate, as objects of the understanding, two things indissolubly combined by early and constant habit, as objects of the imagination. The silence of modern metaphysicians on this head is the more surprising, that D’Alembert long ago invited their attention to it as one of the most wonderful phenomena in the history of the human mind. “The bias we acquire,” I quote his own words, “in consequence of habits contracted in infancy, to refer to a substance material and divisible, what really belongs to a substance spiritual and simple, is a thing well worthy of the attention of metaphysicians. Nothing,” he adds, “is perhaps more extraordinary, in the operations of the mind, than to see it transport its sensations out of itself, and to spread them, as it were, over a substance to which they cannot possibly belong.” It would be difficult to state the fact in question in terms more brief, precise, and perspicuous.

That the illusion, so well described in the above quotation, was not overlooked by Descartes and Malebranche, appears unquestionably, from their extreme solicitude to reconcile it with that implicit faith, which, from religious considerations, they conceived to be due to the testimony of those faculties with which our Maker has endowed us. Malebranche, in particular, is at pains to distinguish between the sensation, and the judgment combined with it. The sensation never deceives us; it differs in no respect from what we conceive it to be. The judgment, too, is natural, or rather (says Malebranche), it is only a