The Universe of Discourse
Tue, 04 Mar 2008

"Boolean" or "boolean"?
In a recent article I wrote:

... a logical negation function ... takes a boolean argument and returns a boolean result.
I worried for some time about whether to capitalize "boolean" here. But writing "Boolean" felt strange enough that I didn't actually try it to see how it looked on the page.

I looked at the the Big Dictionary, and all the citations were capitalized. But the most recent one was from 1964, so that was not much help.

Then I tried Google search for "boolean capitalized". The first hit was a helpful article by Eric Lippert. M. Lippert starts by pointing out that "Boolean" means "pertaining to George Boole", and so should be capitalized. That much I knew already.

But then he pointed out a countervailing consideration:

English writers do not usually capitalize the eponyms "shrapnel" (Henry Shrapnel, 1761-1842), "diesel" (Rudolf Diesel, 1858-1913), "saxophone" (Adolphe Sax, 1814-1894), "baud" (Emile Baudot, 1845-1903), "ampere" (Andre Ampere, 1775-1836), "chauvinist" (Nicolas Chauvin, 1790-?), "nicotine" (Jean Nicot, 1530-1600) or "teddy bear" (Theodore Roosevelt, 1858-1916).
Isn't that a great paragraph? I just had to quote the whole thing.

Lippert concluded that the tendency is to capitalize an eponym when it is an adjective, but not when it is a noun. (Except when it isn't that way; consider "diesel engine". English is what it is.)

I went back to my example to see if that was why I resisted capitalizing "Boolean":

... takes a boolean argument and returns a boolean result.
Hmm, no, that wasn't it. I was using "boolean" as an adjective in both places. Wasn't I?

Something seemed wrong. I tried changing the example:

... takes an integer argument and returns an integer result.
Aha! Notice "integer", not "integral". "Integral" would have been acceptable also, but that isn't analogous to the expression I intended. I wasn't using "boolean" as an adjective to modify "argument" and "result". I was using it as a noun to denote a certain kind of data, as part of a noun phrase. So it is a noun, and that's why I didn't want to capitalize it.

I would have been happy to have written "takes a boolean and returns a boolean", and I think that's the controlling criterion.

Sorry, George.

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