Certainty isn't needed. Heck, they don't even have to know you've fixed anything. What you can't have is them focusing on the chance that it won't work - regardless of what they think about how likely it is to work.
Is that quote complete enough? First, you
used the word "Certainty", I did not.
We are following very similar lines. Your saying that you don't want them to focus on doubting that it will work. Ok, I am taking the positive side approach and saying they must believe it WILL work. How is that, that different? I think our views are along the same line as the question, "Is the glass half full or half empty?"
If the client doesn't know that you "fixed" something, why would they ever want to come back to you? I have the clients see themselves in the future as successfully overcoming their problems which reinforces their chances of success.
I believe in the client-oriented model. I don't "fix" anything. I provide the client with the means and opportunity to work out the problem within themselves. After all, they are the ones doing all the work.
I had a client tell me she thought she had totally wasted their money after a session because she didn't feel any different. When confronted with her problem situation the next day, she discovered, yes, it did work.
The 4 mental attitudes just tell you which thought process is most likely to succeed and which are likely to fail. They are not absolutes by any means.