What René Descartes really meant when he said Cogito Ergo Sum

Cogito Ergo Sum. I think, therefore I am. You may have heard this quote too many times to count, but what does it really mean? The simplicity of the phrase conceals the deeper philosophical truth that René Descartes was trying to convey in his Discourse on the Method, where the phrase first appeared in French as je pense, donc je suis. The Latin version, cogito ergo sum, would be used in his synthesis Principles of Philosophy, and became one of the core tenets of modern philosophy. The best way to fully understand its meaning is to look at the questions that Descartes asked himself in order to arrive at this realization.

How do I know if I really exist?

If you aren’t sure if you’re a real person or just a construct in The Matrix, then congratulations. You’re at the same place that Rene Descartes was four hundred years ago. He too was consumed by the idea that we could not know anything for certain. You couldn’t trust your own senses, because they could deceive you. What you see with your own eyes could be an optical illusion, things can be distorted through lenses and liquids, objects in the distance are smaller than they appear up close. The same applies to other senses like sound, touch and smell.

Descartes realized that he could experience all those sensations in a dream, even though they weren’t real, so how could he be sure of them when he was awake? The entire fabric of your reality, everything you see, hear and feel could be created by some entity just to trick you. This “demon” that Descartes imagined was very much like the machines in The Matrix, capable of completely controlling every aspect of perception and manipulating the senses.

Defeating doubt with doubt

So if nothing was real, and everything an illusion, how did Descartes discover that he actually existed? While he could not be sure of anything else, he realized that as long as he could doubt, it meant that he was capable of thinking, so at the very least he existed as something that was capable of thinking. Or in his own words “I doubt, therefore I am — or what is the same — I think, therefore I am.

A more complete form of the dictum was created by the French poet Antoine Léonard Thomas in an essay praising Descartes, where he wrote dubito, ergo cogito, ergo sum, meaning I doubt, therefore I think, therefore I am. Therefore, if you are able to doubt, that means you can think, and as long as you can think, you exist in some form as a thinking thing, a being capable of thought.

Here is the complete paragraph from Principles of Philosophy, originally published in Latin, where Descartes first used that phrase “ego cogito, ergo sum“.


Sic autem rejicientes illa omnia, de quibus aliquo modo possumus dubitare, ac etiam, falsa esse fingentes, facilè quidem, supponimus nullum esse Deum, nullum coelum, nulla corpora; nosque etiam ipsos, non habere manus, nec pedes, nec denique ullum corpus, non autem ideò nos qui talia cogitamus nihil esse: repugnat enim ut putemus id quod cogitat eo ipso tempore quo cogitat non existere. Ac proinde haec cognitio, ego cogito, ergo sum, est omnium prima & certissima, quae cuilibet ordine philosophanti occurrat.


While we thus reject all of which we can entertain the smallest doubt, and even imagine that it is false, we easily indeed suppose that there is neither God, nor sky, nor bodies, and that we ourselves even have neither hands nor feet, nor, finally, a body; but we cannot in the same way suppose that we are not while we doubt of the truth of these things; for there is a repugnance in conceiving that what thinks does not exist at the very time when it thinks. Accordingly, the knowledge, I think, therefore I am, is the first and most certain that occurs to one who philosophizes orderly.

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