Which is correct: “So far as I know” or “As far as I know”?

Question

Which is more appropriate: "So" or "As"?

Answer 1

Both are equally valid because as far as and so far as have exactly the same meaning in this context. However, so far as may be considered slightly less formal, as it short for in so far as.

as far as
to the extent that:
as far as I am concerned, it is no big deal

(in) so far as
to the extent that:
it was a windless storm so far as blizzards go

_NOAD

You will likely often hear so far as I know in casual conversation. However, you will probably only see as far as I know in written/formal contexts. For the extremely formal, in so far as I/we know would not be out of place.

Answer 2

"As far as I know" is the idiomatic expression I'm familiar with.

as far as conj. To the degree or extent that: They returned at nine, as far as we know. Usage Note: As far as the Usage Panel is concerned, as far as had better be followed by both a subject and a form of go or be concerned. As far as is sometimes used as a preposition meaning "as for" or "regarding," especially in speech, but a large majority of the Panel frowns upon this usage. Eighty percent find the as far as construction in this sentence unacceptable: As far as something to do on the weekend, we didn't even have miniature golf. Eighty-four percent reject the sentence The Yankees are still very much alive, as far as the divisional race. Further, 89 percent object to as far as when followed by a noun clause, as in As far as how Koresh got shot, we don't know yet.

_{[The American Heritage® Dictionary of the English Language, Fourth Edition copyright ©2000 by Houghton Mifflin Company. Updated in 2009]}

According to the same source "so far as" is a variation of "insofar as" :

so far as conj. Insofar as: So far as I am concerned, the project is over.

Answer 3

The rule I learned many years ago is that "so" follows a negative. For example, you would say, "She is as tall as her sister, but not so tall as her brother." I suspect this is one of those rules that few are taught these days and because it is rarely used correctly, the incorrect has become acceptable.