Not sure I agree with that. Emphasis on "think" undercuts the point: I wouldn't say that I "think you can't jump over the moon", even though I do not have a formal proof of impossibility handy for that, I'd just say "you can't do that."

In fact, I almost like it better without the word "think" at all: "Whatever can't be done, someone will come along and do it." YMMV, though.