To come to a logical conclusion for, determination about, or solution to something. Well, let's just reason the problem out—there has to be a way to fix this without costing us a fortune. You're wasting your time trying to reason his motivations out. Dan just does things according to his own odd beliefs.

To discuss something with one in a reasonable, logical way in order to achieve an agreement or understanding. Thankfully I was able to reason with the police officer, and he let us off with a warning instead of arresting us. You're wasting your time trying to reason with Dan—he's going to do what he wants,

To attempt to use logical reasoning in order to disprove, counter, or dissuade someone from something. You're wasting your breath trying to reason against his dumb arguments. It doesn't matter how much logic or truth you present, it's still not going to change his mind. I kept trying to reason against the board's decision, but they were determined to follow through with their plan.