There have, historically, been thinkers who have taken the position that nothing is knowable. These skeptics have argued that there is no way to prove that anything is real. Taking this idea one step further, there were the Pyrrhonian skeptics like Sextus
Empiricus. They argued that skeptics went too far when they claimed that nothing could be known. The reason for this is that the claim that nothing can be known is itself a claim to knowledge. So, it is possible to doubt everything because, at least to some thinkers, there can be no absolute proof of the reality or truth of any thing or any idea. While we may think that there should be limits as to what a person can doubt, there is no logical reason that there should be any such limits.