The missing shade of blue and the nature of relation

Famously, Hume objects to his empiricism with the thought experiment of “the missing shade of blue”. After arguing at length that all thoughts and ideas trace back to simple impressions, Hume pauses to raise an objection: shades of a color are clearly simple impressions, but assume you had a sheet of paper where all the shades of blue were laid out as continually getting lighter and lighter, and you cut out a section, taped the ends together, and showed it to someone. Even if they could not see the cut or the tape, they could immediately tell that a shade was missing simply from the break in continuity, and it would also be relatively easy to imagine what color the shade was, even if one had never seen it before.  It’s a testimony to Hume’s genius that he hit on exactly the example showing that human experience of reality cannot be limited to the sensation of the physical world, since relations like “bluer” or “less blue” can be real even where they are not relative to something sensed. We identify a hiatus in the colors continually getting lighter or darker only because we can see their relation to something absent. Assume the missing shade was periwinkle. Then while there is no quality periwinkle among your colors, no quantity of tinting color proportions that gives rise to periwinkle, and no substance of ink with that shade, there is still a real relation of all the colors to periwinkle, which is exactly why we both see it as missing, and why the quality is in fact missing. One of the great virtues of Hume’s argument is how it demonstrates the reality of relations. The colors are not bluer or less blue than an idea of periwinkle any more than the color is missing because we notice a gap. The color is missing because relations are real features of the extra-mental world even without a positive quality, substance or quantity to serve as their foundation.

Hume’s thought experiment generalizes to any continuous order of one thing to another. Assume we cut a section out of the water cycle, say, condensation. If we showed the resulting picture to anyone they could immediately see that something was missing: water evaporated and rose and later fell down in droplets, but there was a missing transition section from one to another. If we drew the process of prenatal development while jumping from the embryonic stage to birth, everyone would see that the fetal stage was left out. True, nature does sometime move in quantized shifts, but this does not undermine our ability to use continuity as a heuristic, and at any rate there is a good deal of work to be done in explaining the meaning of quantized shifts. It is not clear whether the shift arises from trying to model discrete quantities with continuous magnitudes, or whether the absence of any physical meaning to transition states is like the absence of any meaning in a game of checkers to moving a piece halfway onto a black square. At any rate, the point is not to defend the absolute value of continuity, but to use continuity where it really exists to show how relations are the only things real things the extra-mental world whose reality does not depend on being in the extra-mental world. To return to our example: all the shades of blue have the same real, extra-mental relation to periwinkle whether it is present or missing from a palate of shades.

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