In Meditation II, Descartes states, "I am; I exist - this is certain." Explain why Descartes claims that his knowledge here about this cannot be doubted?
Descartes, in a very organized fashion, and with the help of a thought experiment in the form of a demon who is actively deceiving him about everything, he decides to reject everything as completely false. He writes:
"I will suppose, then, that everything I see is fictitious. I will believe that my memory tells me nothing but lies. I have no senses. Body, shape, extension, movement and place are illusions. So what remains true? Perhaps just the one fact that nothing is certain!"
He notes how our senses can deceive us.
"Whatever I have accepted until now as most true has come to me through my senses. But occasionally I have found that they have deceived me, and it is unwise to trust completely those who have deceived us even once."
He observes how people make reasoning mistakes even when they think they have all the information.
"I sometimes think that others go wrong even when they think they have the most perfect knowledge."
He even notices that a very real dream is indistinguishable from real life!
"Often in my dreams I am convinced of just such familiar events – that I am sitting by the fire in my dressing-gown – when in fact I am lying undressed in bed!"
Despite all the examples he could conceive of being deceived, there was one thing in which he could not disprove, nor could the theoretical deceiving demon cause him to be deceived: the fact that he is thinking - that he exists. If he can think the thought "I am, I exist" then he must actually exist; even if he exists in order to be deceived. Said differently, if he doubts he exists, then his very thought that he is doubting his own existence proves that he exists! On this one fact, he can place his complete certainty.
"Thinking? At last I have discovered it – thought! This is the one thing that can’t be separated from me. I am, I exist – that is certain."
He later writes,
"even if I am in a perpetual dream, and even if my creator is doing his best to deceive me? These activities are all aspects of my thinking, and are all inseparable from myself. The fact that it is I who doubt and understand and want is so obvious that I can’t see how to make it any clearer."
In sum, if an entity conceives of the thought, "I exist" then the statement is true and beyond doubt. Even if the entity were to doubt it existed, then it could still be certain it exists because it is an entity actually doing the doubting and, subsequently, exists!
Having established that an entity exists, it becomes free to begin a deep and complex inquiry into why it exists - what its purpose is. And that will become the topic for another day.
Descartes, R. (1639). Meditations on First Philosophy. (A. Blunden, Trans., J. Cottingham, Ed.). Cambridge University Press. https://www.marxists.org/reference/archive/descartes/1639/meditations.htm