Let me be clear. Whenever I hear this phrase, uttered so frequently in our current political discourse, I think reflexively of Sebastian in Twelfth Night: “Go to, go to, thou art a foolish fellow. Let me be clear of thee.” No doubt I now seem arrogant, but the phrase for me is so tainted by the lack of clarity that often follows it, that I cannot break the habit of my thought. And it is a remarkable thing about human thought that, having read this, it may well become a habit of your thought too.

I begin this chapter on fog with this quotation for a simple reason. The relationship between fog and thinking is most obvious in its opposite: when we think without fog, we think clearly. Of all the forms that water takes, however, it is perhaps that of fog which, in English at least, has the oldest connection to thinking. The Oxford English Dictionary gives the earliest occurrence of fog to refer to the meteorological condition as 1544; the figurative usage follows by 1565: “The authorities here alleged are full of Fogge, and false grounde.” Both of these, however, are long preceded by the adjective foggy. From its earliest usages, this word figured a mental obliviousness, as in John Skelton's phrase, c.1487, “Enkankred with the foggy mystis of clowdy ignoraunce.”