The missing shade of blue

by Massimo Pigliucci

This semester I’m teaching a graduate level course on “Hume Then and Now,” which aims at exploring some of the original writings by David Hume, particularly the Enquiry Concerning Human Understanding, and contemporary philosophical treatments of Humean themes, such as induction, epistemic justification, and causality.

I want to talk here about a particular episode belonging to Section 2 of the Enquiry, where Hume introduces the famous problem of the missing shade of blue, which is still discussed today in philosophy of mind. I think reflection on the problem itself, as well as some attempts to reconcile what appears to be a glaring contradiction in Hume’s own treatment of it, tells us something interesting about how philosophy is done, and sometimes overdone.

To set the stage, let me tell you a bit about the broader Humean project first. Hume proposed nothing less than an overhaul of the way we do philosophy, largely in reaction to what he (correctly, in my mind) perceived as the useless and obscure musings of “the schoolmen” who preceded him and who were still influential at the onset of the 18th century.

A cardinal point of Hume’s novel approach to philosophy was going to be to conduct, as the title of the book clearly states, an inquiry into how human beings understand things, because only by appreciating human epistemic limits can we produce sound philosophical reasoning. (This approach still inspires plenty of philosophers today, and even a number of scientists, such as social psychologists Jonathan Haidt and Joshua Greene.)

Hume explicitly and very clearly sets out his program for his readers in Section 1 of the Enquiry, appropriately entitled “Of the different species of philosophy.” But it is Section 2, “Of the origin of ideas,” that concerns us here.

It begins with the introduction of Hume’s famous distinction between ideas and impressions. Ideas are thoughts, while impressions are sensations. The first are derived from memory and abstract thinking, the second from the senses. Ideas, Hume argues, are (weaker) “copies” of impressions, and impressions are obtained directly from experience.

For instance, we can feel love for someone (an impression) and we can think about the concept of love (an idea). Clearly, says Hume, the feeling is much stronger than the concept, as expected if it were derivative. Ultimately, according to Hume, all knowledge comes from experience, which is why he is classified among the British empiricists, like Locke (as opposed to the continental rationalists, like Spinoza and Leibniz) — even though Hume actually had a fairly low opinion of Locke, whom he saw as still confused by the influence of the schoolmen.

Hume acknowledges that it would seem that human imagination is boundless, as we can think about all sorts of things that don’t actually exist (and cannot therefore be experienced), such as unicorns and gods. But he then argues that no matter how apparently fanciful our imagination is, all our complex ideas are in fact combinations of simpler ones, and those in turn can be traced to our experience.

Take god, for instance: “The idea of God, as meaning an infinitely intelligent, wise, and good Being, arises from reflecting on the operations of our own mind, and augmenting, without limit, those qualities of goodness and wisdom.”

Hume gives two arguments in support of his thesis: first, whenever we analyze a complex idea — such as that of god — we find that it is, in fact, traceable to a combination of simpler ones, of which we ultimately have direct experience (we have all seen intelligent, wise, and good people). Second, we know that when people have a defect in their sensorial perception they are incapable of forming the corresponding ideas: a blind man has no concept of color, because he has never had an impression of what a color feels like through his senses.

And now we come to the problem caused by the missing shade (for those of you who are following the original text, this is #16 of Section 2 of the Enquiry). I’ll let Hume’s beautifully clear prose speak for itself here (the italics are mine, and they will come in handy during the discussion below):

“There is, however, one contradictory phenomenon, which may prove that it is not absolutely impossible for ideas to arise, independent of their correspondent impressions. ... Suppose, therefore, a person to have enjoyed his sight for thirty years, and to have become perfectly acquainted with colours of all kinds except one particular shade of blue, for instance, which it never has been his fortune to meet with. ... Now I ask, whether it be possible for him, from his own imagination, to supply this deficiency, and raise up to himself the idea of that particular shade, though it had never been conveyed to him by his senses? I believe there are few but will be of opinion that he can: and this may serve as a proof that the simple ideas are not always, in every instance, derived from the correspondent impression; though this instance is so singular, that it is scarcely worth our observing, and does not merit that for it alone we should alter our general maxim.”

Okay, so what’s the big deal, you say? Well, it doesn’t take a sophisticated Hume scholar to figure out that our hero here goes through the following strange sequence: 1. He comes up with what he says is a general principle concerning human understanding; 2. He finds an exception to that principle; 3. He the discards that apparently damning finding as not worthy of consideration. And this despite the fact that Hume had told his readers a bit earlier that the new philosophy he is proposing is subject to empirical disconfirmation, just like the natural philosophy (aka, science) by which it is inspired! What’s going on here? Plenty of commentators have tried to figure it out, attempting to rescue Hume from an embarrassing self-contradiction. After all, this is arguably the most influential philosopher ever to write in the English language. Could it be he didn’t notice that he had successfully refuted his own cardinal doctrine, on which his entire philosophical work is based?

In the following we will look at three of several possible solutions to Hume’s blue dilemma, as summarized in a nice paper by John Nelson (published in 1989 in Hume Studies). I will then argue that there is a good chance that Nelson (and others) over-analyzes things, which is typical of, ahem, analytical philosophy. The answer may be much simpler, more satisfying, and more in synch with Hume’s own conception of “moral” philosophy as analogous to natural philosophy (“moral” at the time indicated all of philosophy other than science, not just ethics). And I will accomplish all of this while at the same time showing just how sensible Hume really was!

The first suggestion advanced by Nelson (rather informally, since he says he overheard it from a colleague…) is that Hume deliberately weakened his empiricist position, giving an opening to the rival rationalist approach through a sort of self-created Trojan horse. Admitting that the missing shade of blue could be conceived a priori, i.e. without recourse to experience, would, in fact, do just that. Now, why would Hume shoot himself in his philosophical foot? Because Hume’s philosophy also includes an important role for instincts (which in fact he discusses right after the section we are concerned with here), and instincts are innate, i.e. they precede direct sense experience. Nelson, however, immediately discards this possibility for the explanation of the missing shade’s problem. If that were really Hume’s intent then he would have constructed his subsequent arguments in the Enquiry in a much more rationalist-friendly fashion, which he most certainly didn’t do.

Option two, then. This one comes from R. Cummins, who proposed it back in 1978 (in philosophy things move slowly, as you know). Essentially, the suggestion is that one can reasonably interpret Hume’s “having an idea of X” (say, the missing shade of blue) as meaning “having a capacity to recognize X,” in which case the apparent contradiction would instantly disappear, since Hume wouldn’t be providing an example that potentially undermines his main thesis, he would simply entertain the possibility that people are capable of recognizing that there is a missing shade of blue among a range of colors offered to them. The problem with this “solution,” as Nelson quickly points out, is that Hume himself is very clear that he considers the missing shade to be a “contradictory phenomenon,” which is entirely inconsistent with Cummins’ way out of the dilemma.

What then? Nelson has his own theory, of course. (Are you still with me? I promise, there will be a pay off, coming up shortly…) This one is subtle and clever. Indeed, I think, too subtle and clever. Nelson essentially suggests that Hume’s bringing up of a possible contradiction to his main thesis about how people form ideas (ultimately, from experience) is in perfect harmony with his even more general thesis that human understanding of “matters of facts” (i.e., everything outside of math and logic) is only probable, never certain. You see the twist? Hume, according to Nelson, is providing a (possible) example of how his own theories about a particular matter of fact — the ultimate origin of human ideas — could be mistaken, which proves his meta-point about there being no such thing as certainty about anything empirical. Very clever, very elegant, and very likely an unnecessary overreach on Nelson’s part.

My humble opinion — since I’m not a Hume scholar — is simply that we need to take Hume at his words. Re-read, if you please, the passages by him that I quoted above, and pay particular attention to the italicized parts: “it is not absolutely impossible,” “simple ideas are not always, in every instance,” “this instance is so singular, that it is scarcely worth our observing,” “does not merit that for it alone we should alter our general maxim.”

Did you see it? Hume simply found a hypothetical example (it would actually be very difficult to do the experiment, if you think about it) that doesn’t go well with his general account. But he thinks that the alleged exception is so contrived as to fail to make a general point, and he therefore (wisely) proceeds to ignore it.

This attitude is similar to the one unreflectively adopted by practicing scientists, and philosophers in general — especially those of the analytic tradition — could benefit from imitating Hume more often. Too many philosophers seem to think that when they find an apparent exception to a general concept, no matter how unlikely or artificial, they have “defeated” the general notion, that exotic counter-examples provide knock-out arguments against a given thesis. But in reality this is generally not the case, and philosophers should just relax about it.

For instance, you may recall my discussion of so-called “Gettier cases” in the context of a treatment of the concept of knowledge. Ever since Plato, knowledge has been defined as justified true belief. Then came Edmund Gettier, who in 1963 published a paper showing that there are instances of what one should consider knowledge and that yet do not seem to agree well with Plato’s definition (read my original post if you are interested in the details). I do consider this an example of (minor) progress in philosophy, because as a result of Gettier-type cases we now have a more nuanced understanding of what counts as knowledge and why. But it can easily be shown that all Gettier-type exceptions to Plato’s concept of knowledge fall into a very narrow category, and they are all very highly contrived. What would a good scientist do, when faced with such narrow anomalies? Very likely precisely what Hume did: ignore them, at least provisionally, and focus instead on the general account to see just how much it can explain before having to be refined or expanded.

It should not come as a surprise, then, that the highly sensible David Hume, whose project was precisely to turn “moral” philosophy into something more akin to natural philosophy (i.e., science) would adopt the pragmatic approach that is so effective in the latter practice. If only more contemporary philosophers were more Humean in spirit I think the whole discipline would greatly benefit. As Hume himself put it, when he happened to be temporarily overwhelmed by a hopelessly complex philosophical problem, “I dine, I play a game of back-gammon, I converse, and am merry with my friends; and when after three or four hours’ amusement, I wou’d re turn to these speculations, they appear so cold, and strain’d, and ridiculous, that I cannot find in my heart to enter into them any farther.” Cheers!