Social Research: An International Quarterly

the author of the book of proverbs was selective in revealing what he did not know. "There are three things that are too wonderful for me, yea four that I know not: the way of an eagle in the air; the way of a serpent upon a rock; the way of a ship in the midst of the sea; and the way of a man with a maid" (Proverbs 30:18).

Here I shall pass over the first three of these unknowables, and touch only briefly on the fourth. I want to address another unknowable phenomenon of nature that many philosophers and scientists continue to declare too wonderful for them: the way of conscious sensation.

Imagine yourself looking at a bright red wall. The red sensation you are now experiencing is unknowable in two fundamental ways. The first is privacy. The sensation is yours alone. No one else can know "what it's like" for you, just as you could not know what it's like for anyone else. The second is explanatory opacity. There is no known explanation for how this conscious experience can arise from the physics of your brain.

It was John Locke, in his Essay of 1690, who first explicitly brought the problem of privacy to scholarly attention.

If the idea that a violet produced in one man's mind by his eyes were the same that a marigold produces in another man's, and vice versa … this could never be known: because one man's mind could not pass into another man's [End Page 157] body, to perceive what appearances were produced.

([1690] 1975, §II.xxxii.15)

But Locke was articulating a problem that it hardly requires a great philosopher to recognize. We have probably played with versions of the problem since we were children, raising questions in the playground to tease and annoy our school friends. How do I know you see red like I do? How do I know if you're really in pain? What if your smells are like my sounds? Ha ha, there's no way of knowing!

Ha ha, indeed. But the conundrum also has a worrying side to it. You may well remember how strange and even upsetting it was at first discovery: not merely an interesting tease, but potentially a serious challenge to the need to share and understand. If we cannot even tell what seeing red is like for someone else, what can we tell?

Imagine. We are walking together in the woods after the rain, dappled sunshine filters through the dripping leaves, a blackbird sings, and the scent of honeysuckle permeates the rich air. It may be that shared moments like this are needed to give meaning to our lives. What then when we realize how little we are truly sharing, that, if truth be told, each of us is merely coloring in the other's consciousness as if it were our own?

Still, what is to be done? Realizing, early on, that the privacy of consciousness is a brute fact about the human condition, most of us as we grew up will have put the problem aside. Sometimes philosophical doubts, however cogent in theory, are hardly worth worrying about in practice. Samuel Johnson famously dismissed the Idealist suggestion that the external world is nothing other than an idea in his own head by kicking a stone and declaring, "I refute it thus." Whatever our first anxieties about the limits to our knowledge, our adult selves will probably want to brush them aside with a confident, plain-man's assertion that, in practice, we know all we need to know.

As it happens, Locke himself unexpectedly pulled back from the brink of his own thought experiment about colors. In a passage, little known even to scholars, that follows the famous one quoted above, he wrote: [End Page 158]

I am nevertheless very apt to think that the sensible ideas produced by any object in different men's minds, are indiscernibly alike. For which opinion there might be many reasons offered: but being besides my present business, I shall not trouble my reader with them: but only mind...