I Think, Therefore I Am: Cartesian Skepticism and Foundationalism

Have You Ever?

Have you ever had a dream that felt so real? A dream where you didn’t know you were dreaming until you woke up? If yes, compare how you felt in the dream to how you feel in your awake life. Is there anyway to know that you are not dreaming right now, and that you have never woken up?


Perhaps we could pinch ourselves. If we don’t wake up, then we aren’t dreaming. However, pinching ourselves doesn’t seem to be a fool-proof way to know that we aren’t dreaming. Perhaps we are a brain in a vat, with our neurons connected to a high functioning computer making us feel as though we are living real life. If so, pinching ourselves isn’t going to let us know if we are a brain in a vat or not. However, the one thing that we do know is that we are experiencing the pinching in some way, whether or not we have hands or it is all just in our mind. We are consciously thinking of the pinching, therefore we must exist in some way.

I Think Therefore I Am

I think, therefore, I am is a philosophical claim which proves that regardless of how we exist, we still exist. If you think, you are a thinking thing that exists in some way. It is a foundational knowledge claim that cannot be refuted, even in the face of radical doubt.

The History

“I think, therefore, I am” was first purported by 17th century French philosopher Rene Descartes in its Latin form: Cogito, ergo sum. Descartes had become disillusioned with his own knowledge and began to doubt everything he had ever known. He subscribed to foundationalism, which is an epistemological approach, or a theory of knowledge, that attempts to justify all knowledge claims from a secure and certain claim. In order to have secure knowledge, it must rest on firm foundations, like a house. If the house, or system of knowledge, doesn’t have a firm foundation, it is shaky and must be deconstructed and rebuilt. 

To deconstruct his knowledge, Descartes first doubted his senses, because in the past, they had sometimes deceived him. After all, we’ve all been tricked by optical illusions. Therefore, Descartes concluded, our senses may always deceive us. There is no way to know whether the sky is actually blue, or if it is just our eyes playing tricks on us. Further, there is no way to know that you are not the only person in the world, and everyone you see is just a mirage. This is known as solipsism, which is the theory that you can only prove the existence of the self, and no others.

Next, Descartes began to doubt his reasoning. If there’s anything we can be sure of, it seems we should know that 2+3=5. However, Descartes reasoned, it is possible that an Evil Deceiver is controlling our thoughts, and making us think that 2+3=5 even though 2+3=13. Therefore, he concluded, our reasoning may always deceive us. There is no way to know whether 2+3=5, or if it is just an Evil Deceiver tricking us.

At this point, Descartes entered the realm of radical skepticism, or extreme doubt. He had no more house, and no more knowledge. He had no way of knowing that the external world or his reasoning was real or correct, because there was the possibility that an Evil Deceiver existed. Descartes began to doubt everything he ever knew. He doubted his senses, his reality, his reasoning, and finally, his existence. Perhaps he didn’t exist at all. At this point, Descartes had reached rock bottom. He doubted his very own existence. He doubted that he was thinking, and he doubted that he was doubting. However, Descartes realized, in doing so, he confirmed the fact that he is doubting and thinking. No matter what the Evil Deceiver may do, Descartes knew that he was doubting and thinking, therefore he existed. “I think, therefore I am,” Descartes stated. If you think, if you doubt your own existence, this proves your existence.

“I think, therefore I am” is a piece of foundational knowledge that cannot be doubted. It was the secure and firm foundation to Descartes’ system of knowledge, the concrete basement floor of a house. From this solid piece, Descartes attempted to build back his knowledge. 

Applying It

You may recognize Descartes’ Evil Deceiver as the modern brain in a vat archetype present in many science fiction films. These are all forms of Cartesian skepticism, or doubt of the reality of the external world. Although Descartes was able to prove that he himself existed, he had difficulty building the rest of his house from this piece. He attempted to appeal to God, but philosophers have been less than impressed with his argument. Thus, Cartesian skepticism continues to haunt and inspire the minds of philosophers and filmmakers alike. 

Descartes’ radical skepticism leaves many problems for the certainty of our knowledge claims. However, although we may not be able to prove the existence of the external world and other minds and the credibility of our reasoning, for pragmatic and ethical reasons, we must continue to act as though they are real. After all, you shouldn’t walk in front of a car just because you’re not certain that it exists. You’re also not certain that it does not exist! Radical skepticism such as Descartes’ may only prove that each of us as a conscious thinking individual exists, but we must continue to behave as though the people and things with which we interact are real and not just figments of our imagination. At least, at the end of the day, we know that we exist.

Think Further

  1. Do we need certainty to have knowledge?
  2. Are there any other knowledge claims that cannot be doubted?
  3. How can you be sure that there are other minds in the world?


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Learn More

  1. Wireless Philosophy. “PHILOSOPHY – History: Descartes’ Cogito Argument.” Youtube, 12 Dec 2014, https://youtu.be/7iGjiSbEp9c.
  2. Miceli, Charles. “‘I think, therefore I am’: Descartes on the Foundations of Knowledge.” 1000 Word Philosophy, 24 Nov 2018, https://1000wordphilosophy.com/2018/11/26/descartes-i-think-therefore-i-am/.
  3. Newman, Lex. “Descartes’ Epistemology.” The Stanford Encyclopedia of Philosophy, Spring 2019, https://plato.stanford.edu/entries/descartes-epistemology/.