“Cogito ergo sum” or “I think therefore I am” is probably the most famous argument in all of philosophy. But René Descartes perhaps didn’t intend it as an argument at all but instead as a rational intuition of something self-evidently true. This post explains the difference and why Descartes says this.
The “I think therefore I am” quote is often associated with the evil demon thought experiment from Descartes’ Meditations on First Philosophy. The evil demon is a form of radical scepticism where Descartes doubts everything he’s ever seen or heard or thought because it’s possible that his entire experience has just been the work of an omnipotent evil demon that’s been deceiving him:
“I will therefore suppose that, not God, who is perfectly good and the source of truth, but some evil spirit, supremely powerful and cunning, has devoted all his efforts to deceiving me. I will think that the sky, the air, the earth, colours, shapes, sounds, and all external things are no different from the illusions of our dreams, and that they are traps he has laid for my credulity; I will consider myself as having no hands, no eyes, no flesh, no blood, and no senses, but yet as falsely believing that I have all these; I will obstinately cling to these thoughts, and in this way, if indeed it is not in my power to discover any truth, yet certainly to the best of my ability and determination I will take care not to give my assent to anything false, or to allow this deceiver, however powerful and cunning he may be, to impose upon me in any way.”
– Descartes, Meditations on First Philosophy (Meditation I)
However, Descartes argues that, even if this is the case and everything he’s ever seen or heard has been a deception by this omnipotent deceiver, he can at least be sure that he exists because there has to be something that’s being deceived in the first place. The fact that he’s able to think about and doubt his existence proves he does exist. Hence, “I think, therefore I am”.
The thing is, Descartes never actually says “I think, therefore I am” in response to the evil demon hypothesis. The argument never appears verbatim in Meditations. Instead, the famous “cogito ergo sum” / “I think therefore I am” quote comes from Descartes’ earlier work, Discourse on the Method:
“But immediately afterward I noticed that, while I wanted thus to think that everything was false, it necessarily had to be the case that I, who was thinking this, was something. And noticing that this truth – / think, therefore I am – was so firm and so assured that all the most extravagant suppositions of the skeptics were incapable of shaking it, I judged that I could accept it without scruple as the first principle of the philosophy I was seeking.”
– Descartes, Discourse on the Method (Part IV)
Instead, what Descartes actually says in Meditations in response to the possibility of the evil demon is something slightly different:
“I can finally decide that this proposition, ‘I am, I exist’, whenever it is uttered by me, or conceived in the mind, is necessarily true.“
– Descartes, Meditations on First Philosophy (Meditation II)
The difference is “I think, therefore I am” has the structure of an argument – with premises and a conclusion. But, says Descartes in his replies to Marin Mersenne, the cogito shouldn’t be understood as an argument from premises to conclusion like this. Instead, the cogito should be understood as a ‘simple intuition’:
“When someone says ‘I am thinking, therefore I am, or I exist’, he isn’t inferring existence from thought by means of a syllogism; rather, a simple intuition of his mind shows it to him as self-evident. If he had been inferring it through a syllogism, it would have been this:
Everything that thinks is, or exists;
I think; therefore
I am, or exist.
And for this he would need already to have known the first premise ‘Everything that thinks is, or exists’; but what actually happens is that he learns it by experiencing in his own case that it isn’t possible to think without existing.”
– Descartes, Second set of objections to Meditations (Marin Mersenne and others) and Descartes’ replies
The reason for this distinction is that if the cogito were an argument then, as Descartes says, he “would need already to have known the first premise ‘Everything that thinks is, or exists'”. And it’s conceivable that this premise could have been a lie put there by the evil demon.
So, instead, Descartes says the cogito should be understood as “a simple intuition of [the] mind [that is] self-evident.” In other words, “I exist” is just a conclusion – but a conclusion that is self-evidently true and, for Descartes, impossible to doubt.
The philosophy textbook written in plain English!