Divide by two
Short Version: Watch the flash!
Long Version: "Divide by two" has its origins in the above flash which user Choark put together after requesting from Archmage help in putting together his first Philsys character sheet. The result of this request was, apparently, a more confused Choark, and the artistic output you see!
It should be noted...
It should be noted at no point that during the conversation did Archmage ever actually tell Choark to divide anything by two; furthermore, at that point in Philsys' history, no formula for calculating any base stat or substat included a division by two. The only aspect of Philsys at the time that involved division by two was the calculation of earned MP at level-up, and as Choark was a 1st level character (and had a MAG score of 0), there would have been no need to divide anything by two.
As a result, "divide by two!" became Archmage's "catchphrase" despite the fact that Choark invented it and that Archmage had never previously told anyone to divide anything by two.
...BUT! Of late, due to various revisions, Philsys does include calculations in which the player must divide by two.