Why do we say "by all means" when we mean "by any means"?

Solution 1:

To my ear, any means exhorts any one particular method, whereas all means implores the use of all and sundry methods.

Solution 2:

I think of it as being a shortened version of "by all means possible or necessary". The "possible" is implied in the phrase; it is a common and quite old phrase as well.

Here is an excerpt from an article by Ted Nesbitt - 11/10/2006, on Allexperts.com:

This phrase is very peculiar. The original version was "by all ways" and the negative version was "in no ways." [Incidentally, the old English spellings were wayes and wyse for "ways."] Both of these are now considered obsolete. The all ways or all means is the positive view, and it means certainly. The negative version, of course, means "certainly NOT". The Oxford English Dictionary -- the ultimate source for word and phrase origins -- gives the year 1593 as the first written usage of by all means. It appeared in one of Hooker's ecclesiastical treatises.