In his Second Meditation, French Philosopher René Descartes (1596-1650) tries to establish absolute certainty about existence in his famous conclusion Cogito, ergo sum, or "I think, therefore I am."
His Meditations are conducted from the first-person perspective, from Descartes' point of view. He expects, however, 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 of this truth will take place from the first person, or the "I" perspective.
All sensory beliefs had been found doubtful in the previous meditation, and therefore all such beliefs are not 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 don't 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 preposition '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.
Observation about the transcendental nature of this argument are significant for its position in the context of rationalism, the opposing empiricism, and the subsequent idealism, especially the work of Immanuel Kant.
Transcendental arguments are partly non-empirical, often anti-theoretical arguments focusing on necessary enabling conditions either of coherent experience or the possession or employment of some kind of knowledge or cognitive ability, where the opponent is not in a position to question the fact of this experience, knowledge, or cognitive ability, and where the revealed preconditions include what the opponent questions.
A few scholars have observed that the statement "Cogito, ergo sum" can be re-conceived as a transcendental argument: 1) I think; 2) In order to think "I think," it is necessary to exist. 3) Hence, I exist. It meets the criteria for a transcendental argument because it takes a fact about one's mental life as a premise, adds that some extra-mental fact is a necessary condition of the truth of that premise, and concludes that the extra-mental fact holds.
This argument would turn on the claim that the statement, "I do not exist" (or better, the proposition that no one exists) is performatively self-defeating in the sense that the fact of its performance counts as conclusive evidence against the truth. That is what connects the mental fact (I am thinking about whether I exist) to the relevant extra-mental fact (I exist). Regardless of how this argument might fail in some other respect, it presupposes neither verificationism nor idealism in closing the gap between the internal and the external.
In discussion about the historical change in our conception about access to truth, French 20th-century philosopher designates Descartes reasoning as decisive in this process, and calls the transformation, accordingly, Le moment cartésien ("Cartesian moment"). Through the long period of Hellenistic and Roman thought, Foucault explains, the “care for oneself,” or a spiritual transformation reached after a long period of asceticism, is a prerequisite for having access to truth.
In the modern age, by contrast, it is assumed that only knowledge gives access to truth. Descartes attributed the knowledge of the spiritual intellect to human reason itself, Foucault argues, and man’s rational faculty became, for the first time since Antiquity, the chief organ of knowledge. In his Meditations, he puts the self-evidence of the subject’s own existence at the very source of access of being, and makes the motto, “know yourself,” into a fundamental means of access to truth.
As Phillip Sherrard puts it, with the statement Cogito ergo sum “thought breaks with everything but itself, and forms as it were a closed world no longer in contact with anything but itself.” Knowing oneself becomes merely a necessary epistemic, and not moral, condition for gaining access to truth. The link Foucault talks about was finally completed when Kant said: if knowledge has limits, these limits exist entirely within the structure of the knowing subject. In his Critique of Pure Reason, he asserts that features of the subject’s own thinking must be constitutive of the very possibility of knowledge .
Even though Descartes argues that we can attain knowledge of God's existence simply by apprehending that necessary existence is included in the clear and distinct idea of a supremely perfect being, his approach is not even concerned about divine nature of our reasoning ability.
Whereas a medieval thinker would perhaps conclude the certainty of his existence in his feeling, or the ability to love, for Descartes human ratio is the beginning and the ultimate end of our existence. That is why this argument represents such a break with the philosophy and theology of the past and the revolutionary inception of modern philosophy.