When do you say that you're certain?

"You only say you are certain when you are not." (Halliday)

It's true. Despite the fact that they contain an explicit marker of certainty, sentences such as That must be John, That's certainly John, That's definitely John and That's John, no doubt all sound less certain, paradoxically enough, than the modally unadorned sentence That's John.

So, beware of people who say they're certain.