Hi, NB, thanks for the info. Hmmmm. I don’t have a copy of the source article, but a reprint of Prejudices: Second Series, which includes that essay does have the variant I mention:

Explanations exist; they have existed for all time; there is always a well-known solution to every human problem neat, plausible, and wrong.

That variant seems to be what is included all over the place, now that I look at it, which makes me think that’s the “true” quotation, and someone else mangled it to “simple, neat, and wrong” somewhere along the line.

I will make corrections at my first opportunity. Thanks again!