Original statement: The phrase originally appeared in French as je pense, donc je suis in Discourse on the Method by René Descartes.
Latin translation: Cogito, ergo sum. It appeared in Latin in his later Principles of Philosophy
From Wikipedia: Later translated into English as “I think, therefore I am” , so as to reach a wider audience than Latin would have allowed.
As Descartes explained it, “we cannot doubt of our existence while we doubt.”
A fuller version, articulated by Antoine Léonard Thomas, aptly captures Descartes’s intent: dubito, ergo cogito, ergo sum (“I doubt, therefore I think, therefore I am”)
Descartes’s statement became a fundamental element of Western philosophy, as it purported to provide a certain foundation for knowledge in the face of radical doubt. While other knowledge could be a figment of imagination, deception, or mistake, Descartes asserted that the very act of doubting one’s own existence served—at minimum—as proof of the reality of one’s own mind; there must be a thinking entity—in this case the self—for there to be a thought.
One common critique of the dictum is that it presupposes that there is an “I” which must be doing the thinking. According to this line of criticism, the most that Descartes was entitled to say was that “thinking is occurring”, not that “I am thinking”.
Essentially, thought cannot end up being the sole provable thing in existence since it has requirements for its own existence.
St. Augustine was one of the early proponents of similar thinking. Parmenides 5th Century BC also said something similar.
Saiva Siddhantha identifies mind and thoughts as perishing with the body and hence these cannot be associated with the identity of I. The soul is believed to be more subtle than the mind. While energies associated with mental activity can be measured, the soul itself cannot be traced by outside methods.