Cogito, ergo sum
The Latin cogito, ergo sum, usually translated into English as "I think, therefore I am",a(https://en.wikipedia.org/wiki/Cogito,_ergo_sum#cite_note-better_translation-1) is the "first principle" of René Descartes's philosophy.
Descartes arrives at it in the Meditations on First Philosophy (1641) by doubting everything that can be doubted, including the evidence of the senses and the existence of the external world (see Evil demon). What survives the doubt is the doubting itself: there must be something doing it.
My position
The cogito is the only candidate I know of for a claim held with certainty, and it is the first of My Presuppositions: I exist. Even so, I hold it with some scepticism. Given how much we feel certain about and should not, it is prudent to be wary of intuitions, even this one. A more careful statement is "there is thinking", since the step from thinking to a persisting "I" that does the thinking is a further assumption.