I've just encountered this quote while searching for 'would do well not to' expression.

"So near is falsehood to truth that a wise man would do well not to trust himself on the narrow edge." — Marcus Tullius Cicero 

It's hard for me to understand what it's saying, but is it because it's out of context? But this one is selected as a "quote" at a quote site.

It seems to me that the that-clause is an advice because of something that the main clause says. OK. I think first of all, I don't understand what the main clause is saying, and this is because I don't understand how the that-clause is functioning here..