The Second Meditation is subtitled "The nature of the human mind, and how it is better known than the body" and takes place the day after the First Meditation. The Meditator is firm in his resolve to continue his search for certainty and to discard as false anything that is open to the slightest doubt. He recalls Archimedes' famous saying that he could shift the entire earth given one immovable point: similarly, he hopes to achieve great things if he can be certain of just one thing. Recalling the previous meditation, he supposes that what he sees does not exist, that his memory is faulty, that he has no senses and no body, that extension, movement and place are mistaken notions. Perhaps, he remarks, the only certain thing remaining is that there is no certainty.
Then, he wonders, is not he, the source of these meditations, not something? He has conceded that he has no senses and no body, but does that mean he cannot exist either? He has also noted that the physical world does not exist, which might also seem to imply his nonexistence. And yet to have these doubts, he must exist. For an evil demon to mislead him in all these insidious ways, he must exist in order to be misled. There must be an "I" that can doubt, be deceived, and so on. He formulates the famous cogito argument, saying: "So after considering everything very thoroughly, I must finally conclude that this proposition, I am, I exist, is necessarily true whenever it is put forward by me or conceived in my mind."
The Meditator's next question, then, is what this "I" that exists is. He initially thought that he had a soul, by means of which he was nourished, moved, could sense and think; and also that he had a body. All these attributes have been cast into doubt, except one: he cannot doubt that he thinks. He may exist without any other of the above attributes, but he cannot exist if he does not think. Further, he only exists as long as he is thinking. Therefore, thought above all else is inseparable from being. The Meditator concludes that, in the strict sense, he is only a thing that thinks.
The cogito argument is so called because of its Latin formulation in the Discourse on Method: "cogito ergo sum" ("I think, therefore I am"). This is possibly the most famous single line in all of philosophy, and is generally considered the starting point for modern Western philosophy. In it, the Meditator finds his first grip on certainty after the radical skepticism he posited in the First Meditation. The cogito presents a picture of the world and of knowledge in which the mind is something that can know itself better than it can know anything else. The idea that we know our mind first and foremost has had a hypnotic hold on Western philosophy ever since, and how the mind can connect with reality has ever since been a major concern. In this conception, the mind ceases to be something that helps us know about the world and becomes something inside which we are locked.
We should note, however, the distinction between the "I think, therefore I am" as stated in the Discourse on Method and the formulation we get in the Meditations: "So after considering everything very thoroughly, I must finally conclude that this proposition, I am, I exist, is necessarily true whenever it is put forward by me or conceived in my mind." Neither "therefore" nor "I think" appear in the Meditations. The absence of "therefore" is important, since it dissuades us from reading the cogito as a syllogism, that is, as a three-step argument as follows:
(1) Whatever thinks exists