TITLE: Most helpful heuristic?
QUESTION [13 upvotes]: What's the most useful piece of mathematical "folk wisdom" you've encountered? I'm talking here about things that aren't theorems, or even conjectures, or even shadows of conjectures -- just broad analogies or slogans that nonetheless tend to be incredibly useful for formulating conjectures, understanding when a line of attack on a problem will or won't work, etc.
The first example that leaps to my mind is the Cramer probabilistic model of the primes, which tends to give astoundingly good predictions for all sorts of things despite being horribly simple.
Oh, right: Community wiki, one tidbit of wisdom per post, please!

REPLY [2 votes]: Every set or function you can build in a concrete fashion starting from (countablye many) other measurable sets or functions is measurable.
There are some counterexample to this. But it gets you a feel for measurability like we have a feel for what is continuous and what is not.