Great IdeasKnowing and thinking
How we know anything
What knowledge is, and how it differs from guessing or belief.
The question
What is knowledge, and where does it come from: from reason, from the senses, or from both?
- 1 Plato 375 BC
- 2 Aristotle 340 BC
- 3 René Descartes 1641
- 4 John Locke 1689
- 5 George Berkeley 1710
- 6 David Hume 1748
- 7 Immanuel Kant 1781
- 8 Bertrand Russell 1912
- Mainly from reason
- Mainly from the senses and experience
- From both working together
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.
c. 375 BC
Plato, The Republic, Books VI–X
Mainly from reason
Plato pictures prisoners in a cave who see only shadows on a wall and take them for real things. The senses, he says, show us shadows; real knowledge is of unchanging truths, which the mind reaches by reasoning. Education is the slow climb out of the cave into the sunlight.
c. 350–322 BC
Aristotle, Metaphysics
From both working together
Aristotle begins with the senses: we love seeing even when it is useless. Experience builds up from many memories, but knowledge proper means grasping the reasons and causes behind things. Unlike Plato, he looks for the general truths inside the things we see.
“All men by nature desire to know.”
1641
René Descartes, Meditations on First Philosophy, with the Objections and Replies
Mainly from reason
Descartes sets aside everything that could possibly be doubted, including the senses. What survives is that he is thinking, so he must exist. From there he tries to rebuild knowledge on ideas so clear and distinct that reason cannot doubt them.
“I am, I exist, is necessarily true each time it is expressed by me, or conceived in my mind.”
1689
John Locke, An Essay Concerning Human Understanding
Mainly from the senses and experience
Locke denies that we are born with any ideas built in. The mind starts like blank paper, and all its ideas come from experience: from the senses and from noticing our own thinking. Knowledge is then the mind seeing how its ideas agree or disagree.
“To this I answer, in one word, from experience: in that all our knowledge is founded, and from that it ultimately derives itself.”
Go to the rung: Locke: An Essay Concerning Human Understanding
1710
George Berkeley, A Treatise Concerning the Principles of Human Knowledge
Mainly from the senses and experience
Berkeley pushes Locke further. If all we ever know are our ideas, he asks, what sense does it make to speak of matter existing apart from any mind? For him, to be is to be perceived, and the steady order of the world comes from God.
1748
David Hume, An Enquiry Concerning Human Understanding
Mainly from the senses and experience
Hume divides knowledge into two kinds: relations of ideas, as in math, which are certain, and matters of fact, which rest on experience. We expect the sun to rise because it always has, but reason cannot prove it will. Habit, not logic, carries us from the past to the future.
“Custom, then, is the great guide of human life.”
Go to the rung: Hume: An Enquiry Concerning Human Understanding
1781; second edition 1787
Immanuel Kant, The Critique of Pure Reason
From both working together
Kant says Hume woke him from his slumber. He agrees that knowledge begins with experience, but argues that the mind supplies its own forms, such as space, time and cause, that shape all experience. Neither the senses nor reason can give knowledge alone.
“Thoughts without content are void; intuitions without conceptions, blind.”
1912
Bertrand Russell, The Problems of Philosophy
From both working together
Russell begins by asking whether anything can be known for certain. He separates knowing things directly, by acquaintance, from knowing about them through descriptions. He holds that some general truths, such as logic, are known by reason, while most of what we know rests on the senses.
“Is there any knowledge in the world which is so certain that no reasonable man could doubt it?”
For young readers
How do you know your dog is in the yard? You can see him and hear him bark. That's knowing by your senses. How do you know 2 + 2 = 4? You can figure it out in your head. That's knowing by reason.
For a long time, thinkers argued about which way of knowing comes first. Many ended up saying we need both, like two eyes working together.
Talk about it
- Name three things you know because you saw them, and three you know because you figured them out.
- What's the difference between knowing something and just believing it?
- In Plato's cave, the prisoners think shadows are real. Have you ever been fooled by your eyes?
- Hume says we expect the sun to rise because it always has. Is that good enough reason?
- How could you find out if something you read is true?
Where else on the site
- Deductive and Inductive Arguments
- Strong and Weak Inductive Arguments
- Belief and Doubt
- Observing Like a Scientist
- Arguments from Analogy
Every rung tagged with this idea:
- The Syntopicon and Mortimer Adler
- Plato: Phaedo
- Plato: Meno
- Plato: The Republic, Books VI–X
- Plato: Theaetetus and Sophist
- Aristotle: Logic
- Aristotle: Metaphysics
- Bacon: The Advancement of Learning
- Bacon: Novum Organum
- Descartes: Discourse on the Method
- Descartes: Meditations on First Philosophy
- Locke: An Essay Concerning Human Understanding
- Berkeley: The Principles of Human Knowledge
- Kant: The Critique of Pure Reason
- Goethe: Faust
- Bergson: An Introduction to Metaphysics
- Russell: The Problems of Philosophy
- Heidegger: What Is Metaphysics?
- Bohr: Atomic Theory and the Discussion with Einstein
- Heisenberg: Physics and Philosophy
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.