Persuasion becomes manipulation when you stop respecting the other person’s agency. Persuasion says,
“Here is what I think, here are my reasons, and here is the evidence.”
Manipulation tries to control the outcome with thinking errors. It can use deception, selective facts, emotional pressure, fear, guilt, or other tricks designed to push someone toward a conclusion without giving them a fair chance to think it through for themselves.
The line is not always clean. All persuasion involves influence. We choose which facts to present, how to frame them, and which arguments we think are strongest. Emotion is not automatically manipulation either. A moving story can be perfectly honest. A useful test is to ask:
Am I helping this person understand my position, or am I trying to bypass their ability to decide for themselves?
If you have to hide facts, distort the opposing view, or knowingly mislead them to win, you have crossed the line.
Roger Williams gives us a good model. He argued hard and clearly believed other people were wrong, but liberty of conscience meant they still had the right to remain unconvinced. That is an important limit on persuasion. Make your case. Ask questions. Challenge ideas. Present the facts. But eventually, let the other person decide.