How we change what others think, feel, believe and do
The simplest assertion is just to say you are right and they are wrong. If you speak with enough passion and certainty, it is surprising how often this method can work, even when the other person clearly knows more than you.
I'm right about this. I know what I'm doing.
Yes, of course it's true. It's obvious.
Assert special knowledge
If simple assertion is not enough, you can claim to have access to particular information or know things that they do not. Say you have personal experience or know someone who has critical knowledge. Say you have sat at the feet of the gurus and been admitted to the inner circle. Point out how you know things that nobody else knows.
Well I've spoken personally with Richard about this and let me tell you that he says it is right.
I know this because I was there. Were you?
Indicate that you are an expert in the field or are superior to the other person in some other way. Say you are qualified or have studied the subject. Note that you are recognized as an expert in this field. Talk about the papers you have written, the conferences presented to and so on.
I've done a degree in the subject. I should know.
Ask Jim Bowen if you think I'm wrong. He asks for my help all the time.
It is surprising how often you can get away with saying things if you do it with confidence and an air of knowing you are completely right. Just saying 'I'm right' implies others are wrong and can cause them to doubt their own thoughts.
This is something that politicians and various pundits do well. They appear expert and pronounce on matters where they actually have relatively little understanding. But because they sound like they know what they are talking about, then other people accept this.