Descartes said, "Cogito, ergo sum(I think, therefore I am)". I agree with him on the fact that this is the only thing that we can ever truly know for sure. Is there anyone who could tell me if there is anything else we could ever know for certain?
Descartes' famous Cogito is a very clever answer to his hyperbolic doubt, but it has had critics all along.
A very prominent critic is Bertrand Russell who claims in chapter II of his The Problems of Philosophy that the "I" in "I think, therefore I am" is not suffiently warranted. Descartes probably conceived of the Cogito in French, and he first wrote it in Latin, but both versions have the same vulnerability.
Russell's problem is that there is no certainty that the "I" remains identical to itself over time because it is known through experience and not a priori.
So, one can never be certain that the I that exits now is the same as the I that will exist, or did exist at any time other than now. Worse, the existence of the I is an inductive inference from various sense-data and is only probable. So, the Cogito, as stated cannot be known to be true a priori, as Descartes claims.
An earlier critic, David Hume, argues in An Enquiry Concerning Human Understanding (Sect. XII, Part III) that "The existence, therefore, of any being can only be proved by arguments from its cause or its effect; and these arguments are founded entirely on experience. If we reason a priori any thing may appear to produce any thing."
But these are empirical perspectives from which nothing can be known to a certainty. Descartes' own rationalist perspective seems replete with some necessary truths, such as that an effect cannot exceed its cause in any way because of the inviolability of Ex nihilo, nihil fit (a further assumed certainty).
However, Descartes' own device of the evil genius doubt seems entirely insurmountable. He claims that the evil genius could never bring it about that he is nothing when he thinks he is something, but that claim contradicts the definition of omnipotence.
Well, the obvious, the death of your current physical (gross) body is the only guarantee in life.
I came across some relevant information, although it insists you believe that mathematics holds to a standard of certainty:
Additionally, if you are familiar or have any interest in mathematics, I'd recommend checking out these mathematical phenomena below. You may certainly enjoy trying to rationalize them: