One of the most prominent questions in the philosophy of knowledge since very early in epistemic thought is the question of scepticism (or skepticism).
Scepticism asks: what do we really know? Our various sources of knowledge are known to be fallible. We can be fooled by illusions or deception, we can reason poorly and our memories can fail us. What reason have we to assume that anything we perceive is actually true? Even supposedly scientific claims about the present and future seem to be fallible.
Common questions of sceptics include the following: