And just when you thought....

....therefore you were....

 Meditations by Rene Descartes

Rene Descartes did not just arbitrarily come up with the phrase "I think, therefore I am." Or for you Latin buffs "cogito ergo sum."  
Many people, however, may not realize the lengths to which Descartes went in order to arrive at this assertion. You may be quite amazed at his process...

 --  Although he believes in creation himself, he wants to prove to others that God and the soul exist, and to do so he starts from the complete opposite end - think of it as a kind of process of elimination...but backwards. Descartes first has to prove that he himself exists, in some form. He is attempting to honestly and truly approach the problem, and establish first what it is that he can be certain of. --

Meditation I: Establish the things that are in doubt. The question of Perception...what can be proven as "real." In effect, nothing except the mind. 

• The senses are deceiving. We may have all been tricked by our senses.
• We may be dreaming. In a dream, we have no way of knowing that we are dreaming until we awake. Is it not plausible then that we are in an elaborate dream, but we merely think that we are awake?

  ---Question: If everything is doubtful to exist, and we may be in a state of deception, is God so deceiving that he would trick us into believing that things are real when they are not?

  ---Answer: God is all good by definition, then it cannot be He that is deceiving us.

Therefore it must be an evil demon possessing us all, and convincing us that this is real. 

"I will suppose, then ...that some malignant demon, who is at once exceedingly potent and deceitful, has employed all his artifice to deceive me; I will suppose that...all external things, are nothing better than the illusions of dreams, by means of which this being has laid snares for my credulity; I will consider myself as without hands, eyes, flesh, blood, or any of the senses, and as falsely believing that I am possessed of these; I will continue resolutely fixed in this belief, and if indeed by this means it be not in my power to arrive at the knowledge of truth, I shall at least do what is in my power, and guard with settled purpose against giving my assent to what is false, and being imposed upon by this deceiver, whatever be his power and artifice."

This really freaks him out...the next part sounds somewhat like "The Matrix."

"...this undertaking is arduous, and a certain indolence insensibly leads me back to my ordinary course of life; and just as the captive, who, perchance, was enjoying in his dreams an imaginary liberty, when he begins to suspect that it is but a vision, dreads awakening, and conspires with the agreeable illusions that the deception may be prolonged."

He's wondering if he should have taken the blue pill, but continues the next day.

Meditation II: Descartes realizes that all things are perceived in effect through the mind. In other words, despite all else, he is established as a thinking entity. He cannot be convinced, persuaded or deceived that he does or does not exist, without existing in the first place.

"I had the persuasion that there was absolutely nothing in the world, that there was no sky and no earth, neither minds nor bodies; was I not, therefore, at the same time, persuaded that I did not exist? Far from it; I assuredly existed, since I was persuaded. But there is I know not what being, who is possessed at once of the highest power and the deepest cunning, who is constantly employing all his ingenuity in deceiving me. Doubtless, then, I exist, since I am deceived; and, let him deceive me as he may, he can never bring it about that I am nothing, so long as I shall be conscious that I am something. So that it must, in fine, be maintained, all things being maturely and carefully considered, that this proposition I am, I exist, is necessarily true each time it is expressed by me, or conceived in my mind."

Then he shows that we know more of the mind than we do the body...

       "...there is nothing more easily or clearly apprehended than my own mind...I am a thinking (conscious) thing, that is, a being who doubts, affirms, denies, knows a few objects, and is ignorant of many,-- [who loves, hates], wills, refuses, who imagines likewise, and perceives; for, as I before remarked, although the things which I perceive or imagine are perhaps Nothing at all apart from me [and in themselves], I am nevertheless assured that those modes of consciousness which I call perceptions and imaginations, in as far only as they are modes of consciousness, exist in me."

This is how Descartes comes to the conclusion that has become legendary, "I think, therefore I am." He goes on with four more Meditations after this, and I invite you to read them if you liked this sample.
Rene Descartes really has a way with words...and thoughts.

Source: http://oregonstate.edu/instruct/phl302/texts/descartes/meditations/meditations.html