“I think therefore I am” or “Cogito, ergo sum” is a dictum, which is coined by Rene Descartes in his work Discourse of Methods 1636. Descartes poses that there is an immeasurable uncertainty in this world and the only certain thing, which is beyond any doubt, is the thinking process of a person.

Descartes starts with the uncertainty of the senses. Human beings have five senses: sight, hear, touch, taste, and smell. For ages, theses sense has been considered as the true source of attaining knowledge. However, Descartes took the responsibility of challenging the reliability of these senses. There are innumerable cases in which we are deceived by our senses. Sometimes we see things, which do not really exist.  In the dark you may not clearly identify the objects, you may misunderstand someone’s statement because of the distortion, and your taste buds may not function properly because of your unhealthy condition. We experience various sense perceptions’ failures in our everyday life. Therefore, the reliability of sense perception is indeed questionable.

Descartes also shatters the difference between the state of sleep and awake. He believes that may be we are always in a state of sleep. The things we saw, the experiences we have are maybe just our dreams. The reality, which we believe in, is may be a long unending dream. Therefore, it can be said the human beings are living in a continues delusional state.

Reaching to the extreme point of skepticism, Descartes suggest a way out from the life of illusions. The only thing that services his methodic doubt is the process of thinking or the process of skepticism itself. According to Descartes, you can doubt anything, but you cannot doubt that you doubt and if you doubt it means that you are thinking. Therefore, thinking becomes the only indubitable thing. For this reason, Descartes concluded that the pillar of the certainty of existence is reasoning. Hence, he concludes, “I think therefore I am”.

This dictum of Descartes had ushered the new era in the field of philosophy. It challenged all the prior beliefs and shifted the importance from the sense perception to the reasoning. Reasoning, for Descartes, is the only thing, which is unquestionable.