@Hef: Well, we
do know that minds are brains. There has as of yet been no compelling evidence to suggest that any of the processes of the "mind" are
not carried out by the brain.
Regardless, our knowledge of how the brain works is admittedly limited. We know, for example, what the greater role of each section of the brain is, and how these sections interact with each other. (Even with only this knowledge, there is tremendous evidence that the workings of the "mind" really just take place in the brain -- for example, damage to specific parts of the brain can cause people to act extremely immorally or to believe irrationally that their parents are impostors.) We know, from a high-level perspective, that brain cells (neurons) fire in such a way as to excite nearby neurons, and that this process, repeated countless times by the billions of neurons in the brain's network, is responsible for all the wonderful things the brain does. But that network is so utterly huge and complex that it will be a while until, for example, we have a very in-depth understanding of how the brain is able to represent concepts like "chair" and "love" or perform high-level reasoning. Regardless, again, just because we don't currently know how the brain does something doesn't mean it doesn't do it.
So, if the question is
Rather than that, wouldn't it be better to default to "We don't know" than to insist that something you don't, in fact, know is the truth?
then the answer is that there is indeed a lot we don't know and that scientists are willing to acknowledge it when there is still work to be done. Asserting that something you don't know is in fact the truth is not a scientific thing to do at all; that practice lies more within the realm of religion.