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100 The two following remarks, which I shall state with all possible brevity, appear to me to go far towards a solution of the problem proposed by D’Alembert.

1. According to the new theory of vision, commonly (but, as I shall afterwards shew, not altogether justly) ascribed to Dr Berkeley, lineal distance from the eye is not an original perception of sight. In the meantime, from the first moment that the eye opens, the most intimate connection must necessarily be established between the notion of colour and those of visible extension and figure. At first, it is not improbable that all of them may be conceived to be merely modifications of the mind; but; however this may be, the manifest consequence is, that when a comparison between the senses of Sight and of Touch has taught us to refer to a distance the objects of the one, the indissolubly associated sensations of the other must of course accompany them, how far soever that distance may extend.

2. It is well known to be a general law of our constitution, when one thing is destined, either by nature or by convention, to be the sign of another, that the mind has a disposition to pass on, as rapidly as possible, to the thing signified, without dwelling on the sign as an object worthy of its attention. The most remarkable of all examples of this occurs in the acquired perceptions of sight, where our estimates of distance are frequently the result of an intellectual process, comparing a variety of different signs together, without a possibility on our part, the moment afterwards, of recalling one single step of the process to our recollection, Our inattention to the sensations of colour, considered as affections of the Mind, or as modifications of our own being, appears to me to be a fact of precisely the same description; for all these sensations were plainly intended by nature to perform the office of signs, indicating to us the figures and distances of things external. Of their essential importance in this point of view, an idea may be formed, by supposing for a moment the whole face of nature to exhibit only one uniform colour, without the slightest variety even of light and shade. Is it not self-evident that, on this supposition, the organ of sight would be entirely useless, inasmuch as it is by the varieties of colour alone that