“One does what one is; one becomes what one does.”

I've been told this many times in the wrong way of course.

This is fun. Essentially, this can be abbreviated ( and perhaps is supposed to be ? ) as a single ( confusing ) sentence.
Later: Hm; I tried, but am unable to [ sensibly ] re-position the duplicative "does" and the "is" and "becomes." Too many tenses, but ought to be workable. Maybe it would make a good parlor game. "Put these three truths in one sentence."
Well, it was fun trying ( but "trying" isn't in the list ! )

may be, could be, sometimes

You are what you eat.
Garbage in, garbage out. ;-)