Understanding "I think, therefore I am"

“I think, therefore I am.” In 1637, Descartes penned what has become the most oft-quoted catch phrases from epistemology, if not all of philosophy. Compare the phrase to other philosophical catch phrases, like the Golden Rule in ethics or the “We hold all men to be created equal” in politics. In my experience, and it was certainly true of me, there is not the same understanding of what Descartes meant when he wrote the phrase like there is for what the teachers of the Golden Rule meant or what the Founders meant with “We hold all men to be created equal.”

This piece will give the reader a modern epistemological context for understanding what Descartes penned in 1637.


What does it mean when you say you know something? What type of thing can even be known? These are basic questions, yet they will prove to be important. Bring up in your mind a piece of knowledge that you have, something very basic, let’s say, “I am reading a blog post on Applied Sentience.” You see the computer screen in front of you with the webpage open and the blog post front-and-center, and on the basis of this you come to know that you are reading a blog post. This is a perfectly natural use of the word. Therefore, at least a portion of this type of thing that comes to be known can be expressed as a proposition, and it can either true or false.

What can be said of this proposition that you know? If you stopped reading Applied Sentience and yet you, for some odd reason, held on to your knowledge about reading a blog post, would it be appropriate for knowledge? The intuition is a resounding no. Therefore, it seems that it is important that for you to have knowledge with respect to any proposition, it is important that the proposition be true. We retract our claims to knowledge quite a lot with statements like, “I just knew that my team was going to win, but alas, I did not.” It is also absurd to know propositions that cannot be true, like, “It is raining and it is not raining.”

What relationship must we hold with regards to this true proposition? There are an uncountable number of true propositions, but there is something different between any given true proposition and the knowledge we hold. It is exactly that, in fact, it is that we do “hold” the proposition. You believe that you are reading a post on Applied Sentience. If you did not believe it, then it would be odd and inappropriate for you to claim you had knowledge. In fact, claims of this variety are members of Moore’s Paradox.

And so we have a true belief, but is knowledge anything more? There are sorts of true beliefs you could hold, and still fall short of knowledge. For instance, you could accidentally predict the coming lottery by use of chicken bones. It is unlikely, but it is also possible, so it is worth our consideration. Why do chicken bones which yield true beliefs not confer appropriate knowledge? Because they are inappropriate justification. Notice that in our running example, you believe the truth on the grounds that you see a computer screen with a blog post open.

With a few challenges to our intuition, we have come conclusion to what most modern and ancient epistemologists think is at least sufficient for knowledge: justified true belief.


Descartes knew of this formulation of knowledge, but he thought it was unsatisfiable with regards to the typical cases we would call knowledge. Descartes would say that you do not have knowledge even of our simple proposition, that you are reading a blog post. What Descartes noticed that made him think this is that you cannot discount the possibility that you are being deceived. The way that Descartes presented this concern is by saying that it is always at least possible that an Evil Demon is causing you to experience a blog post in front of you, but in fact it is just an illusion. Notice that this means that you can have a belief with an appropriate level justification, but Descartes was nervous that despite this, our experiences cause beliefs that are not true.

This is what is called a skeptical argument, as it leads to skepticism where we intuit that we have knowledge. Descartes would say that there is no level of justification which entitles you to think your belief is true with regards to sensory data, as they cannot be infallible. Descartes still wanted to be able to form justified true beliefs about the world, however, and he especially wanted to have knowledge of mathematics, science, and God. His struggle was to find a system which was both infallible against the Evil Demon and possible for human agents. He examined all of his beliefs, searching for any that survived the Evil Demon. He noticed that there was one belief that the Evil Demon could not possibly deceive him of!

“I think, therefore I am.” Applying our intuited standards for knowledge, we see that would be very hard to actually think while not believing that you think. The justification that one has for their thinking is personal to the person alone, which is what makes it immune to the Evil Demon. The truth of one’s existence is something we cannot access, but it would be very hard for us to reject our own existence. This argument applies only to the first-person perspective, as Descartes would say that there is no way for you to know of the thoughts of others without the Evil Demon interfering.

It is on this foundation that Descartes restored his belief in science, mathematics, and God. Descartes wanted the same obviousness and necessity in his other beliefs as he had with his “I think, therefore I am.”

Is it true?

Does knowledge require infallibility? Do you need to be able to discount all of the possibilities to know that the sun will rise, that you have a birthday, or that the White House exists? Descartes may have thought so, but with this initial exposition into the analysis of knowledge, you are more armed than ever to find your own answers to these questions.