Rereading Hume’s Enquiry has brought the so-called “missing shade of blue” problem to my attention again. I have never accepted that it is a problem, and while I was driving yesterday, I thought of a few ways to demonstrate this.
The problem is as follows. Hume’s theory of perception classifies all perceptions as either ideas or impressions. Impressions come from sense experience; ideas come from impressions. This theory holds as long as no ideas can be generated without the use of an impression. However, Hume lists an apparent exception: imagine a man who has lived his entire life having seen all the different shades of blue save one. If shown a palette of all the shades of blue that he is familiar with, placed in order, will he be able to detect the absence of a shade? The common-sense answer is yes – and yet Hume dismisses it as a minor if singular expection. Several camps exist on this issue – one holds it really is a exception, and another does not, but it’s not easy to reconcile either position with Hume’s line of argument.
I can think of several reasons that Hume was right to dismiss this objection, though he probably should not have been as mysteriously cavalier about the matter, especially given the rhetorical aims of the Enquiry.
Some of the following suppositions match preexisting arguments. I have placed them in order from weakest to strongest.
1.) The situation as given is impossible to replicate. Color is not made of separate shades, but rather a continuum. How can the man be sure he has not seen that shade before? Did a team of scientists keep him in a bubble for his entire life that was drained out that particular shade? They would have to make sure he had never seen a prism or a rainbow. It’s like saying the man has used numbers all his life without ever encountering 42. Could Hume’s man perceive a missing shade without the presentation of all the shades of blue? Probably not. The example is loaded – it assumes, in fact, that there is a missing shade, a problem I’ll address a bit later.
2.) If I accept the situation, the idea of the missing shade is still not independent of impression – it requires extensive knowledge of color, which is dependent on simple sense perception. One individual color on the entire spectrum does not constitute an idea independent of sense perception, especially if defined as a blend of two colors. Furthermore, the mere notice of a gap in the sequence is built on a foundation of years of experience with color, and the concept of a gap itself is not necessary to mathematics that I know of. This argument is a little too ordinary-language philosophy for me, but it’s important nonetheless.
3.) The perception of a gap in a series, or in any pattern, does not require that gap to actually exist. This, I feel, was Hume’s plan all along – his coming evisceration of causality would render the blue-shade example moot.
Much of the argumentative strength of the blue-shade example comes from our knowledge that there IS a missing shade, but the man in the example does not have that certainty. He only suspects there is one – he cannot prove it, for he has no sensory experience of it. Rather, he can only suggest there is a very high probability there is a missing shade, much like I can only posit there is a very high probability that the Indian Ocean exists (I’ve never seen it) until I have seen it, and even then I may be misled, for our senses are rather untrustworthy.
In the 1985 film Goonies, the Mikey character finds the skeleton of a long-dead pirate called One-Eyed Willy, who is still wearing an eyepatch. Curious, Mikey pulls back the eyepatch… and there is no eye socket, only bone. The expectation was that the patch covered something absent, but Willy never had an eye to begin with. The anticipated missing eye is revealed as non-existent. This line of thinking leads rather quickly to Schrodinger’s cat; it is not until the moment of sense perception that questions of existence or non-existence can be partially enlightened.
Imagine this scenario – Hume’s blue-deprived man perceives there is a missing shade, and goes looking for it… and never finds it. He experiments with dyes, travels the world, gives talks to breathless audiences, writes furious monographs. He dies without ever seeing it or without any human being ever finding it. Some scholars, in fact, suggest his perceived “gap” in the color spectrum is actually a fundamental principle of the color spectrum, evidence of a limitation of the human eye, or a mere symptom of the man’s madness-tinged brilliance.
Hume’s notion of causality allows such a scenario, as it allows ALL scenarios. The mere notice of a possible missing shade demands nothing. Take John Couch Adams’s predictions of the existence of Neptune. What if they had come to nothing? Newton’s laws would have to be reexamined. What if, rather, the measurements made by Bouvard had been incorrect, and there were no discrepancies in the data upon remeasurement? The perception of a gap or discrepancy in a pattern is a sense perception that requires absolutely nothing to follow it. This of course does not require that nothing does – only that deductive logic is useless for such questions.
But, you might, ask, can’t Hume’s colorist have an IDEA of a missing shade that is independent of sense perception without it having to exist? Well, no. I once worked in a eyeglass lab with a color-blind fellow, who oddly enough was very good at color dying lenses; he went solely by darkness of tint and the labels on the dye vats. He knew there was an entire world of color that he did not have access to, and I’m positive he thought about what it might be like on many occasions, but he had nothing save dark/light patterns – anyone who has experienced color knows it is far more than that – and word labels to go on. They could give him an ‘idea’ of what he could not experience, but he has no independent way of confirming if his ‘idea’ matches up, save the unreliable testimony of six billion people or so. His ‘idea’ cannot approach a color-sighted person’s ‘idea’ of that shade (which is itself imperfect in proportion to experience with that shade); it is at best an approximation made up of similar sensory perceptions that he does have access to.