QuestionsCan we be certain of anything?What do I know? Neither senses nor reason can judge themselves
Can we be certain of anything?We must doubt

What do I know? Neither senses nor reason can judge themselves

What do I know?; Montaigne's motto; Montaigne's scepticism

Michel de Montaigne 1533 · 1592'Each man calls barbarism whatever is not his own practice': our customs are not the measure of everything.
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What it claims

We can affirm nothing with certainty: the only honest attitude is to weigh reasons without tipping, asking "What do I know?"

Why

The senses give only appearances, and reason can only be checked by another reason, endlessly.

The same idea through history
Sextus Empiricus190Montaigne draws a large part of the Apology from Sextus Empiricus, whom he read: the "wheel" (the circle) and the infinite regress come from the Greek sceptic's modes.
Michel de Montaigne1580In the Apology for Raymond Sebond (Essays II, 12), Montaigne makes "What do I know?", set under a balance, his motto. The balance weighs opposing reasons without tipping.
In short

"What do I know? Neither senses nor reason can judge themselves" is an argument by Michel de Montaigne (Essays II, 12, Apology for Raymond Sebond (1580), 1580) on the question "Can we be certain of anything?" (answer: we must doubt). Its claim: We can affirm nothing with certainty: the only honest attitude is to weigh reasons without tipping, asking "What do I know?". According to AskPhilosophy's logic check (Z3 solver), the argument is valid only if one grants a premise the author leaves unstated (P4). AskPhilosophy presents 2 objections, 0 of them with a documented reply, and the original text behind each premise, checked word for word.

Essays II, 12, Apology for Raymond Sebond (1580)
Valid with an implicit premise

Montaigne had a balance in equilibrium and the question "Que sçay-je?" engraved on a medal. It is not "I know nothing", which would already be a claim. It is a question left open. In the longest part of his Essays, the Apology for Raymond Sebond, he attacks the pride of reason. The reasoning below is a reconstruction: Montaigne does not write it as numbered premises, but one can follow it in the famous passage on the "wheel".

P1To judge a thing, we need an instrument of judgement we can be sure of: our senses, or our reason.
P2Our senses give us not the things themselves but their appearances, which change with our state (health, age, illness, sleep): so they cannot decide.
P3As for reason, no reason is established without another reason: to verify it we need another instrument, then another, without end.
P4 · implicit premiseWe have no instrument of judgement other than the senses and reason.
Therefore we can affirm nothing with certainty: the only honest attitude is to weigh reasons without tipping, asking "What do I know?"

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 P4, one can accept P1 to P3 and find another support: for example an immediate self-evidence (like "I think") or agreement between independent instruments. Many philosophers say this is exactly what science does.
The argument is valid: P1 sets that a secure instrument is needed, P2 and P3 that the two available instruments are not secure, P4 that there is no other. The weak points are P4 (are intuition, science, shared experience further instruments?) and P1 (do we really need an already-verified instrument to judge?). Montaigne adds that his conclusion must be put as a question and not a claim, because "I doubt" at least claims that one doubts.
Analogy. A thermometer giving odd readings: to check it works, you compare it with another thermometer, which would in turn have to be checked against a third. Montaigne says all our knowledge is in this situation.

Objections

2

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

Blaise Pascal 1655 · attacks the conclusion

Pascal admires Montaigne for one thing only: he lowers the pride of reason. But he refuses to stop there. Doubt and certainty destroy each other: the one who establishes certainty and the one who establishes doubt each ruin the truth of the other, so neither can stand alone. For Pascal, only the faith of the Gospel holds man's greatness and weakness together.

no reply
Montaigne never answered this objection.
The debate goes on (our summary) This objection does not refute Montaigne's doubt: it uses it to lead to faith. A sceptic would answer that faith is then one more hypothesis, in the sense of Agrippa's trilemma. Our source is the Conversation with M. de Sacy, reported by Nicolas Fontaine: it is not a text written in Pascal's own hand.
René Descartes 1637 · attacks the conclusion

Descartes also uses doubt, but in order to get out of it. He does not want to imitate the sceptics, who doubt only to doubt and always remain undecided. His aim is to cast aside the loose earth and sand and reach the rock or the clay: to find a firm point. That point will be "I think, therefore I am". Doubt thus becomes a means, not an end.

no reply
Montaigne never answered this objection.
The debate goes on (our summary) Descartes is here describing the sceptics' doubt in general, without naming Montaigne. The sceptic replies that the rock is not so solid: Arnauld objected the Cartesian circle (see the discussion in Agrippa's trilemma), and many readers are not convinced by Descartes's way out. Montaigne could also say that the eagerness to find a firm point at all costs is exactly the pride he fights.