“Cogito, ergo sum” (Latin: “I am thinking, therefore I exist,” or traditionally “I think, therefore I am”) is a philosophical phrase by René Descartes, and it is a translation of Descartes’ original French statement: “Je pense, donc je suis,” which occurs in his Discourse on Method (1637).
The interpretation of this phrase has been subject to numerous philosophical debates. The phrase expresses a skeptical intellectual climate which is indicative of early modern philosophy.
In the First Meditations Descartes explains why he can call his beliefs into doubt, since his beliefs have deceived him before — I think we can all relate to one experience where our beliefs have been totally wrong and we feel the way old Descartes feels here. He argues that perhaps he is currently dreaming or that God is actually a deceiving demon, or that he is simply crazy. This gives him reason to be skeptical of all his beliefs, which leads us into the Second Meditations. Here is where he convinces himself that nothing of the world is real. He essentially disbelieves everything that can possibly be called into question and whittles existence down into nothing.
In the Second Meditation, Descartes tries to establish absolute certainty in his famous reasoning: Cogito, ergo sum or “I think, therefore I am.” These Meditations are conducted from the first person perspective, from Descartes.’ However, he expects his reader to meditate along with him to see how his conclusions were reached. This is especially important in the Second Meditation where the intuitively grasped truth of “I exist” occurs. So the discussion here of this truth will take place from the first person or “I” perspective. All sensory beliefs had been found doubtful in the previous meditation, and therefore all such beliefs are now considered false. This includes the belief that I have a body endowed with sense organs. But does the supposed falsehood of this belief mean that I do not exist? No, for if I convinced myself that my beliefs are false, then surely there must be an “I” that was convinced. Moreover, even if I am being deceived by an evil demon, I must exist in order to be deceived at all. So “I must finally conclude that the proposition, ‘I am,’ ‘I exist,’ is necessarily true whenever it is put forward by me or conceived in my mind. This just means that the mere fact that I am thinking, regardless of whether or not what I am thinking is true or false, implies that there must be something engaged in that activity, namely an “I.” Hence, “I exist” is an indubitable and, therefore, absolutely certain belief that serves as an axiom from which other, absolutely certain truths can be deduced.