its a cold winters day. you see someone out of your window, all wrapped up in a coat and a scarf and a hat. You assume its a person But how do you really know whats under the clothes? It could be a robot...or there could be nothing under there. All human beings base their lives on assumptions, but if the knowledge that those assumptions are based on is incorrect, if human beings are being decieved in some way, then there is no way of proving that anything really exists.