09 Sep

Cogito ergo sum

Cogito ergo sum

Cogito ergo sum, or its translation, “I think, therefore I am,” is a frequently-quoted line from well-known mathematician and philosopher René Descartes.  I’ve noticed that it often seems to be misunderstood; this post is a brief outline of the idea and some of my thoughts on it.

When concluding cogito ergo sum, Descartes was attempting to determine what truths, if any, could be truly known beyond all doubt.  He had a tough time of it, finding that almost everything he could think of was subject to at least some degree of uncertainty.  He imagined a scenario involving a demon playing a trick on the senses – that the world might not really exist, and the demon was magicking up an illusion for him.  A modern equivalent would be the Matrix or being in a computer simulation.1 Today, the term for being sceptical about all things except one’s own existence is known as ‘Cartesian doubt’, an eponymous tribute to Descartes.

Cogito ergo sum was Descartes’ answer to his question of what could be said to be definitively true.  The only thing a person thinking about knowledge can know exists, Descartes concluded, is that they themselves exist, or else there would be no thinking.  Another way to put it is that if we know a subject is performing an action, then we know that the subject must exist (or else they could not be performing the action).2 Think, or rather, experience, feel, etc., are the only actions we know are happening, and they must involve a subject – us (or, more precisely, me and maybe you as well).  So, strictly speaking, cogito ergo sum is not the only truth that can be known; we can also know about the existence of thoughts and feelings: I know that at least one feeling exists because I feel it; I know that at least one thought exists because there is one in my mind; I know that certain sensations exist because I am sensing them, etc.  All of these require subjectivity, therefore there must be a subject (‘me’).  I do not, however, know for certain anything about the nature of myself, except that what I call ‘I’ is an entity that is, in some sense, experiencing.3

Though it is a relatively simple idea, I’ve seen some misunderstandings I’d like to address.  One is a quite forgiveable misconception about the meaning of ‘I’ in this context.  It doesn’t mean who you think you are – all of that could be a lie told to you by this demon.  I am not necessarily the author of this blog post, I just think that I am.  I don’t know for sure that this blog exists, or that I am not asleep in some vivid dream, or that I am not a conscious imagining of a more intellectually-capable being.  I don’t know that the past existed, or that the future will exist.4  All I know is that to feel requires subjectivity, and therefore a subject must exist to feel them.5

Another misunderstanding is that I cannot say, “you think, therefore you are.”  I don’t know that you think – you only appear to me to think.  I could be the only conscious creature in the whole world – the rest of the world’s population the imagining of the demon or produced by the computer simulation.  I don’t know that the world doesn’t disappear every time I close my eyes.  Because I don’t know for sure that anyone else thinks, I can’t conclude that there must be some entity (that I am referring to as ‘you’) doing the thinking.6

I haven’t seen the following misunderstanding personally, but I’m aware that people don’t like being told they might not exist, so I will address it also: Some people might think that Cartesian doubt is an extremely arrogant proposition.  Supposing that it is – of course, that doesn’t speak to its truth value.  It’s simply the admission that if I was in the Matrix, I’d have no way to tell, and therefore I cannot rule it out.  It’s just the way it is, so I think it would be wasteful to call it arrogant, and doing so would not address any arguments in favour of it (but would, however, expose the accuser as being biased to one preferred truth over another).

The final misunderstanding I’d like to mention is one that I came across today.  A person asked (half-jokingly, I imagine), “I sometimes don’t think because I have an afternoon nap, so can I say that I sometimes don’t exist?”  It’s readily apparent that there is something fishy with the use of logic here, and unless a person is already proficient in logical arguments, they might have difficulty pinning down what it is.  The fallacy in operation is known as denying the antecedent, and it works like this: There is an argument that goes “x, therefore y“.  From that argument, it does not follow that “not x, therefore not y“.  In this case, x is the antecedent.  In other words, if you say, “I think, therefore I am”, it doesn’t follow that, “I don’t think, therefore I am not.”7

Cogito ergo sum is to me a very interesting idea, from which a lot of strange and unlikely thoughts can come from.  If you’re the type to enjoy this kind of philosophy, maybe you’ll find it enjoyable to consider solipsism, Cartesian doubt and whether you can think of anything else that you can know to be true – on the other hand, I could understand if you rathered not to think about it.


  1. In philosophy, this is concept is known as a brain in a vat

  2. Logically speaking, we can say, “if thinking requires a thinker and there exists thinking, then there exists a thinker,” although it’s actually simpler than that.  If it’s true that “I am thinking“, then we can easily identify the axiom, “I am.” The statement “I think therefore I am” is actually tautologous, because we know all we need to know from the first half, “I think.”  This problem does not afflict the Latin version however, which might be more precisely translated as: “(of oneself) thinking, therefore am”.  Stated in Latin, it more closely follows the form of a statement of logical identity (i.e., A is A

  3. We could, for example, be manifestations of a shared consciousness, or any number of seemingly unlikely (but unprovable) possibilities. 

  4. See my previous post on the nature of time. 

  5. All actions require a subject to perform them.  Thinking, however, is the only one that I know is being performed.  If I truly knew that I was writing, I could say ‘I write, therefore I am,’ but I don’t truly know. 

  6. The idea that only one’s own mind can be truly known to exist is called solipsism.  Solipsists are people who deny (or are reasonably sceptical of) the possibility that other people also have consciousness. 

  7. It is, however, possible to make an argument called denying the consequent.  You start with the same argument, “x, therefore y“.  It then follows that “not y, therefore not x” (i.e., I am not, therefore I don’t think). 

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