Great IdeasKnowing and thinking
Doubt and certainty
Questioning what we think we know, and what survives the questions.
The question
How much can we be sure of? Is doubt a danger, or the first step toward knowing?
How the books answer
Oldest first. Each stop is our own short summary of what the author argued; follow the link to the rung for the whole lesson.
written after 399 BC
Plato, Apology
At his trial Socrates explains that the oracle at Delphi called him the wisest man in Athens. Puzzled, he questioned people famous for wisdom and found they didn't know what they thought they knew. His only wisdom, he concluded, was knowing that he did not know.
1580–1595
Michel de Montaigne, Essays
Montaigne took as his motto the question 'What do I know?' He piles up examples of how customs, opinions and even the senses differ from place to place. His doubt makes him modest and tolerant, and he still lives and writes with great enjoyment.
c. 1600
William Shakespeare, Hamlet
Hamlet doubts the ghost, his mother, his friends and himself. He stages a play to test whether his uncle is guilty. Shakespeare shows doubt as both a mark of a thoughtful mind and a trap that can freeze a person from acting.
1605
Francis Bacon, The Advancement of Learning
Bacon says hasty certainty is the enemy of learning. If we start with certainties we end in doubts, but if we are willing to start with doubts, careful inquiry can lead to certainty. Doubt, used well, is a tool.
“So it is in contemplation: if a man will begin with certainties, he shall end in doubts; but if he will be content to begin with doubts, he shall end in certainties.”
1641
René Descartes, Meditations on First Philosophy, with the Objections and Replies
Descartes doubts on purpose, as far as doubt can go. He imagines a powerful deceiver making him wrong about everything. Yet even then, he says, he must exist in order to be deceived, and that certainty becomes his starting point.
“I will suppose, then, not that Deity, who is sovereignly good and the fountain of truth, but that some malignant demon, who is at once exceedingly potent and deceitful, has employed all his artifice to deceive me…”
1748
David Hume, An Enquiry Concerning Human Understanding
Hume finds that we cannot prove by reason that the future will be like the past, or even see causes at work. But he does not think we should doubt everything. Nature makes us go on believing, and he recommends a mild, modest doubt that keeps us careful.
Go to the rung: Hume: An Enquiry Concerning Human Understanding
1902
Henri Poincaré, Science and Hypothesis
Poincaré warns against two lazy habits: doubting everything and believing everything. Both, he says, save us the trouble of thinking. Science needs careful judgment about which parts of a theory are facts, which are conventions and which are guesses.
“To doubt everything or to believe everything are two equally convenient solutions; both dispense with the necessity of reflection.”
1912
Bertrand Russell, The Problems of Philosophy
Russell starts with a table: it looks different from every angle and in every light, so what is the real table? Philosophy may not settle such questions, he says, but it frees us from the tyranny of custom and widens our sense of what is possible.
“Is there any knowledge in the world which is so certain that no reasonable man could doubt it?”
For young readers
To doubt is to wonder if something is really true. It's good to doubt a little. If a stranger online says a magic pill will make you fly, you should doubt it!
But you can't doubt everything, or you'd never do anything. Good thinkers ask questions, check the evidence and then decide. Socrates said the first step to being wise is knowing what you don't know.
Talk about it
- What is something you used to believe and now doubt?
- Socrates said he was wise because he knew he didn't know. What did he mean?
- Is there anything you are completely sure of? How do you know?
- When does doubting help you? When does it get in the way?
- Poincaré says believing everything and doubting everything are both lazy. Do you agree?
Where else on the site
Every rung tagged with this idea:
These pages are our own short synthesis, written for families. They were inspired by Mortimer J. Adler's Syntopicon, the index of great ideas in volumes 1 and 2 of Great Books of the Western World, but the questions, summaries and groupings are ours, not the Syntopicon's. Short excerpts come from public-domain translations, credited where they appear.