The words below are not solely mine. A great portion of it comes from Bertrand Russel's work in "The Problems of Philosophy", rephrased a little here and there, with some additional comments from me. Very unoriginal, I know. But who cares, right? "good artists copy, great artists steal". :)
Is there any knowledge in this world that is so certain that no reasonable man could doubt it. It's very important for us to distinguish between appearance and reality. Take an example of color and texture of a table. The color changes if we see the table in a different shade of lights. And the table looks smooth if we see it with our eyes, but what if we were to look at it through a microscope? Wouldn't the surface look differently? With mountains and what-not? And if we cannot trust what we see through a microscope, why then should we trust our eyes?
Mr Russel wrote that appearance is the sign of reality. According to him, we shouldn't be sceptical about the existence of what we see, but only the reality behind it. The appearance signifies something behind it, but we can never be sure if the reality is really there. If there's nothing or nobody playing magical tricks on our eyes.
Descartes used a method of systematic doubt, in which he would believe nothing which he didn't see quite clearly and distinctly to be true.
There's no logical impossibility that the whole of life is a dream. As all knowledge must be built upon our instinctive beliefs, therefore there's no reason to reject a belief except on the ground of some other belief.
What about religion then? Why would a Christian, Muslim, Buddhist, or Hindu believe so much in their own religion? What's the foundation for beliefs that sometimes can't be reasoned? What about emotion? Can we reason with emotions? Feelings? Can we?