I Think, Therefore I Am

We cannot doubt of our existence while we doubt.

That we cannot doubt of our existence while we doubt, and that this is the first knowledge we acquire when we philosophize in order.
Cogito, ergo sum is a philosophical statement that was made in Latin by René Descartes, usually translated into English as “I think, therefore I am”. The phrase originally appeared in French as je pense, donc je suis in his Discourse on the Method, so as to reach a wider audience than Latin would have allowed. ~ Wikipedia

“I think; therefore I am” was the end of the search Descartes conducted for a statement that could not be doubted. He found that he could not doubt that he himself existed, as he was the one doing the doubting in the first place. In Latin (the language in which Descartes wrote), the phrase is “Cogito, ergo sum.”

Descartes’ Epistemology
First published Wed Dec 3, 1997; substantive revision Fri Feb 15, 2019
René Descartes (1596–1650) is widely regarded as the father of modern philosophy. His noteworthy contributions extend to mathematics and physics. This entry focuses on his philosophical contributions in the theory of knowledge. Specifically, the focus is on the epistemological project of his famous work, Meditations on First Philosophy. Descartes circulated the Meditations to other philosophers for objections and comments. He responded with detailed replies that provide a rich source of further information about the original work. Indeed, six sets of objections and replies were published with the first edition of the Meditations (1641), and a seventh set was added with the second edition (1642).
Stanford Encyclopedia of Philosophy → Click here

René Descartes
Many elements of Descartes’s philosophy have precedents in late Aristotelianism, the revived Stoicism of the 16th century, or in earlier philosophers like Augustine. In his natural philosophy, he differed from the schools on two major points: first, he rejected the splitting of corporeal substance into matter and form; second, he rejected any appeal to final ends, divine or natural, in explaining natural phenomena. In his theology, he insists on the absolute freedom of God’s act of creation. Refusing to accept the authority of previous philosophers, Descartes frequently set his views apart from the philosophers who preceded him. In the opening section of the Passions of the Soul, an early modern treatise on emotions, Descartes goes so far as to assert that he will write on this topic “as if no one had written on these matters before.” His best known philosophical statement is “I think, therefore I am”, found in Discourse on the Method (1637; in French and Latin) and Principles of Philosophy (1644, in Latin). Rene Descartes → Wikipedia