QuestionsCan we be certain of anything?The cogito
Can we be certain of anything?Yes, through reason

The cogito

René Descartes 1596 · 1650Doubt everything to find what survives doubt.
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What it claims

I am.

Why

Even if I doubt or am deceived, I am thinking; and to think, I must exist.

In short

"The cogito" is an argument by René Descartes (Discourse IV · Meditation II) on the question "Can we be certain of anything?" (answer: yes, through reason). Its claim: I am. According to AskPhilosophy's logic check (Z3 solver), the argument is valid only if one grants a premise the author leaves unstated (P2). AskPhilosophy presents 3 objections, 2 of them with a documented reply, and the original text behind each premise, checked word for word.

Discourse IV · Meditation II
Valid with an implicit premise

Descartes looks for a single truth that cannot be doubted, even if an evil demon were deceiving him about everything.

P1I think: I doubt, I believe I am deceived.
P2 · implicit premiseIn order to think, one must exist: what thinks cannot fail to exist.
Therefore I am.

click a premise to reject it

If you accept every premise, logic forces you to accept the conclusion. So the debate is about the premises.
Checked by a logic solver: the premises cannot all be true while the conclusion is false.
Without the implicit premise: Without P2, the computer finds a possible world where something thinks without existing. Absurd, but nothing in P1 alone rules it out.
As stated, "I think, therefore I am" skips a step: it needs P2, which Descartes himself grants in the Principles (I, 10). With P2 the conclusion follows necessarily. But Descartes insists this is not a multi-step inference: it is an immediate intuition.
Analogy. You can doubt everything you see in a dream: the places, the people, your own body. You cannot doubt that someone is dreaming.

Objections

3

Each objection is a conversation: click to see who replies and where the debate stands.

Gassendi 1641 · attacks premise 1

Why thought? "I walk, therefore I am" would work just as well: any action proves that one exists.

Descartes replies
Descartes
I may dream that I walk without even having a body. Only the awareness of walking, which is a thought, is certain.
T
The "syllogism" reading 1641 · attacks premise 2

If the cogito rests on the general rule "whatever thinks exists", Descartes already knew something before his first certainty. Descartes states this objection himself in order to answer it.

Descartes replies
Descartes
It is not a syllogism: "I think, therefore I am" is grasped as something self-evident. The general rule is learned from this case, not the other way round.
L
Lichtenberg 1790 · attacks premise 1

The "I" is one step too far. We should say "it thinks", as we say "it lightens". That thought belongs to a subject is already an assumption.

no reply
No reply possible: objection raised in 1790, after the author's death (1650).
What today's voices say
Defend it
David ChalmersHe uses Descartes' point: I can doubt the world, but not that I am conscious right now. Consciousness is our primary datum.