I’m what you need to understand, but I’m not something you can simply hand. I’m in a joke, a tale, a text, a debate. Remove me, and your words lose sense.

I plummet low but do not break, and rise but never fly While I myself cannot be seen, it’s my effects you spy With several ways to measure but in truth there’s mainly two I only seem to disappear when I’m the same as you

We check on it when buying used things. We fight for it when we negotiate. We can set it. We often accept it without investigating.

I speak the truth by proving it false, and reveal a lie by making it true. What am I?

Should you have it down to a T, then I guess you have no need for me. If an unsolvable riddle comes your way, that's when my help comes into play. It costs you nothing to unlock me. If you know the answer then leave me be!