What if a demon deceived you about everything?
Descartes imagined a powerful demon devoted to deceiving him in everything. Searching for one thing the demon could not fake, he found it: the fact that he was thinking.
- Reading time
- 4 min
- Difficulty
- Introductory
- Branch
- Knowledge
- First posed
- René Descartes, Meditations on First Philosophy, 1641
The setup
In 1641 René Descartes set himself an extraordinary task. He would, once in his life, doubt everything that could possibly be doubted, and see whether anything was left that was absolutely certain. On that foundation, he hoped to rebuild knowledge.
His senses sometimes deceive him, so he set aside everything he learned through them. He might be dreaming, so even the existence of his own body could be doubted. Then he went further. Suppose there were 'some evil genius not less powerful than deceitful', who devoted all his energy to deceiving him: making him see a world that did not exist, and even making him go wrong when he added two and three.
Under this supposition, what could survive? Descartes found one thing. However much the demon deceived him, he could not deceive him into thinking he existed when he did not, because to be deceived, he had to exist. 'Let him deceive me as much as he will, he can never cause me to be nothing so long as I think that I am something.' So the proposition 'I am, I exist' is necessarily true whenever he thinks it. This became famous in the Latin phrase from his later work: cogito, ergo sum, I think, therefore I am.
“I am, I exist, is necessarily true each time that I pronounce it, or that I mentally conceive it.”
René Descartes, Meditations on First Philosophy, II (1641), trans. Elizabeth Haldane
Why it matters
The cogito is one of the most famous moments in philosophy. It made the thinking self the starting point of knowledge, and shaped centuries of debate about the mind, certainty and the external world.
The demon itself, the idea of total, undetectable deception, is the ancestor of the brain in a vat and the simulation argument.
Key terms
- CogitoShort for 'cogito, ergo sum', I think, therefore I am. Descartes' claim that one thing survives all doubt: the existence of the one who is doubting.
- Method of doubtDescartes' strategy of setting aside every belief that can be doubted at all, in order to find something that cannot.
- FoundationalismThe view that knowledge has a structure like a building: some basic beliefs are justified without depending on others, and everything else rests on them.
- SkepticismIn philosophy, the view that we know much less than we think, or nothing at all, or the practice of doubting claims to knowledge.
- RationalismThe view that reason alone, independent of sense experience, is a major source of knowledge.
The positions
The cogito succeeds
'I think, therefore I exist' is certain, and can serve as a foundation for knowledge.
Certain, but useless
The cogito may be certain, but nothing much can be built on it. Descartes could not get from 'I think' back to the external world without circular reasoning.
The doubt was never real
Descartes' doubt is artificial. Real doubt requires reasons, and the demon gives none.
Arguments and objections
The cogito succeeds
- Best argument
- Any attempt to doubt one's own existence is itself a thought, and so confirms it. This is not an inference from a premise that could be false; it is self-verifying, true every time it is thought. Even the most extreme deception requires someone to be deceived.
- Strongest objection
- Critics from Pierre Gassendi to Georg Lichtenberg asked what entitled Descartes to the 'I'. All that is certain is that thinking is occurring. Lichtenberg suggested we should say 'it thinks', as we say 'it rains'. The step from thoughts to a persisting thinker is not guaranteed.
- The reply
- Defenders reply that every thought is experienced as someone's thought, and that the minimal 'I' of the cogito need not be a persisting substance, only the subject of this very thought.
Certain, but useless
- Best argument
- Descartes tried to rebuild knowledge by proving that a good God exists who would not deceive him, so that his clear and distinct perceptions can be trusted. But he relied on clear and distinct perceptions to prove that God exists. Antoine Arnauld pointed out this 'Cartesian circle' in 1641. Without the bridge, the method of doubt leaves us trapped in our own minds.
- Strongest objection
- Some scholars argue that Descartes' reasoning is not circular, since he only needs the divine guarantee for remembered conclusions, not for present clear perceptions.
The doubt was never real
- Best argument
- Charles Sanders Peirce argued that we cannot begin with complete doubt, since we must start from the beliefs we actually have. Ludwig Wittgenstein argued in On Certainty that doubting requires a background of things held fast, and that a doubt which doubted everything would not be a doubt at all. Some propositions are not known but are the hinges on which inquiry turns.
- Strongest objection
- That we cannot actually live in total doubt does not show that the demon scenario is impossible, or that we know it is false.
Who argued what
- The cogito succeeds René Descartes, Augustine of Hippo
- Certain, but useless Antoine Arnauld, David Hume
- The doubt was never real Ludwig Wittgenstein, Charles Sanders Peirce
Variations
Augustine first
Over twelve centuries before Descartes, Augustine of Hippo wrote, in The City of God: 'If I am mistaken, I exist' (si fallor, sum). Even being wrong requires existing. Descartes' contemporaries noticed the similarity.
The demon and mathematics
Descartes' demon could even make him wrong about simple arithmetic. If you cannot trust that 2 + 3 = 5, can you trust the reasoning that leads to the cogito? Some think this makes the demon too powerful to escape at all.
Go deeper
Where 'cogito, ergo sum' appears
The phrase 'I think, therefore I am' appears first in French ('je pense, donc je suis') in the Discourse on the Method (1637), and in Latin ('ego cogito, ergo sum') in the Principles of Philosophy (1644). In the Meditations themselves, the formulation is instead 'I am, I exist, is necessarily true each time that I pronounce it, or that I mentally conceive it.'
Schools of thought: Rationalism
Related ideas
Further reading and sources
- EncyclopediaStanford Encyclopedia of Philosophy: Descartes' Epistemology
- WikipediaEvil demon
- WikipediaCogito, ergo sum
- BookMeditations on First Philosophy, René Descartes
- BookDescartes: The Project of Pure Enquiry, Bernard Williams
Open questions
- Is there anything you can be absolutely certain of besides your own existence?
- Does 'I think' prove an 'I', or only that thinking is happening?
- Can you build knowledge on a single certainty?