Policy by aphorism is fraught:

"Don't worry about people stealing your ideas. If your ideas are any good, you'll have to ram them down people's throats."

https://en.wikiquote.org/wiki/Howard_H._Aiken

I think this is the guy that gave me the gem.

    If a source is known to lie then the proper weight to place on information from that source is zero.

AKA: You don't try and second guess anything that say, you ignore everything they say.

But it's tempting to assume the information is false, and if you do that, you could be making a big mistake. I assume by "zero weight" you mean you don't assume it's true or false, which is quite distinct.

One way to look at it is that every statement is true, given sufficient context. The question is, how should it be interpreted, and what is it a true statement about? Generally, when a liar says something, it is intended to either make you believe it is true, or make you believe it is false, or confuse you. So there is true information being provided, it's just not the face value of the statement.

By weight I mean the multiplier used when summing it with all the other things you know.

Meaning you don't use information from sources that lie to make decisions. Full Stop.