Here's a place where we can dig into this question still further.
At least some of us think we have a pretty good answer to this question already (we can't know that a statement is true unless it is true, and to be true it has to be logically consistent) but are we all completely satisfied with this answer?
If you'd like a bit of review as to how we got here (and as to how another section of this class came to essentially the same conclusion just a few summers ago), see my online lecture on the course wiki on "Consistency and Knowledge."
So what do you think?
[Lightly edited to enhance readability -TT]