Are you being systematically deceived by an evil demon? Is everything around you an illusion and you are merely a brain in a vat being deceived? That's what Descartes thought and many still believe that today. However, Descartes disproved that theory with one of if not his most famous phrases: "I think therefore I am" (Ergo cogito sum -Latin) This phrase showed that although you can doubt almost everything the one thing you can not doubt is your own existence. For you to doubt your own existence you must exist and therefore you are.