This is a sticking point, and it seems a sticking point that gives many here great problems. The problem is here:

1) EVERYONE knows that nothing can be stated with "absolute" certainty. We could all be brains in a jar--or the modern version of that: all just part of the matrix. To say one can't be a materialist until it's PROVED that all existence is material is...a bugaboo. It's equivalent to saying that you will assume NOTHING until you can know EVERYTHING. Which leads to....

2) If you WISH to navigate the universe and your mind in something like a rational manner, you must--IF ONLY PROVISIONALLY--take a stand somewhere, on something. You can't go anywhere with "Nothing can be known." And as I've stated elsewhere, people who wish to state that "Nothing can be known" really have no good reason for even wanting to be posting things on internet forums (except "nothing can be known" the one thing, ironically, they wish to claim they DO know).