The opposite of “I don’t know” isn’t “I’m certain.”
No, the opposite is, “I’m not sure, but…”
This piece by Seth Godin is great, and is a nice companion to my “It depends” blog. The problem with “I don’t know” isn’t that the person doesn’t know, it’s that they often stop at that point. The same as the “It depends” person. The trick is “…and this is what we’ll do about it.”