We know nothing for certain until we know everything. The next thing that we learn might be that anything or everything we though we knew is false. This is always a possibility no matter how much knowledge we think that we have. This does not mean that we cannot have knowledge only that we can not be certain of it.
Almost anything can be knowledge in the right epistemological system or awarepaducer. This is because an awarepaducer can make any behavior or belief right or wrong within the reality of the awarepaducer. For instance, the strange statement "step on a crack and break your mothers back" in this reality is not true. However, in an awarepaducer or experience machine  produced reality it could be made true. So in this reality this statement is not knowledge but in a different reality it could be knowledge.