How do I know anything about the world around me? By the use of my senses. But I can be deceived by my senses, A straight stick looks bent when it is dipped into water. How do I even know that I am awake, that the whole of reality is not a dream? How can I tell it is not a fabric of delusion woven by some malicious cunning demon simply to deceive me? By a process of persistent and comprehensive questioning it is possible to place in doubt the entire fabric of my existence and the world around me, Nothing remains certain. But in the midst of all this there is nevertheless one thing which does remain certain. No matter how deluded I may be in my thoughts about myself and the world, I still know that I am thinking, This alone proves me my existence, In the most famous remark in philosophy, Descartes concludes: 'Cogito ergo sum'-'I think, therefore I am.