Can we be certain of anything? Philosophers give 4 main answers: yes, through reason, through experience, with care, we must doubt and what escapes words. AskPhilosophy presents 39 arguments, from Thales to Kurt Gödel, reconstructed step by step, logic-checked and verified word for word against their original texts.
The 4 possible answers
Each column is one way of answering the question: what it holds and the philosophers who defend it. Click a column to see only its arguments.
Some truths are absolutely certain, and reason can reach them.
Platon, René Descartes, Kant, Augustin…
All knowledge comes from experience: it can be solid, but never fully certain.
Locke, Ibn Khaldoun, Bacon, Thalès…
Our reasons for belief are weaker than we think: certainty escapes us.
Marx, Socrate, Nietzsche, Al-Ghazali…
The most important truths are lived, felt or shown rather than proved.
Confucius, Lao-tseu, Ibn Khaldoun, Wittgenstein…
The arguments
Click an argument to see it step by step, with the objections and replies.
Yes, through reason
Some truths are absolutely certain, and reason can reach them.
12 argumentsKnowing is having a true opinion and being able to give an account of it
Neither perception nor true opinion is enough: a judge can rule rightly, persuaded by witnesses, without knowing the facts.
The cogito
Even if I doubt or am deceived, I am thinking; and to think, I must exist.
There is certain knowledge that does not come from experience
Mathematics is certain and teaches us something new: this is possible if objects conform to the forms of our mind.
"If I am deceived, I am"
To be deceived, one must exist: even if I am mistaken, I am.
All things are numbers (the Pythagoreans)
Musical scales rest on numbers, which resemble what exists more than fire or water do: so everything would be number.
What is rational is actual
"Real" in the strong sense is not everything that exists: chance and error exist without being real, and what is real is rational.
The chicken and the principle of induction
Experience cannot show that the future will resemble the past without already assuming it: this principle must be accepted as self-evident.
Extending knowledge by investigating things
Everything has its principle and the mind is made to know it: after long study, understanding opens all at once.
Seeing with one's own eyes: the Earth moves
In natural philosophy authority proves nothing: one must judge with one's own eyes, by the senses and reason.
To deny causes is to deny reason
Each thing has a nature from which its actions come; denying causes would destroy knowledge, and we do have certain science.
The book of the universe is written in the language of mathematics
The universe is written in mathematical figures, and what we demonstrate there is known as surely as God knows it.
A perception of mathematical objects: Gödel's intuition
The axioms of set theory force themselves upon us as true, like a perception: there is no reason to trust it less than the senses.
Through experience, with care
All knowledge comes from experience: it can be solid, but never fully certain.
8 argumentsNo innate ideas: the mind is born a blank page
If truths were imprinted on the mind, every mind would know them, yet children do not know even basic logical principles.
Judging a report by the nature of things
An impossible story is not made credible by honest witnesses: it must be checked against how society actually works.
The idols of the mind
The mind distorts what it perceives and keeps only what proves it right; so we need an outside judge: the facts.
The principle of all things is water
The food, heat and seeds of all beings are moist, so water seems to be the origin of everything.
Induction by elimination
A single contrary case rules out a hypothesis; by eliminating whatever is absent when the phenomenon is present, the cause remains.
Know to foresee, foresee to act
Our direct means of acting are weak; knowing laws lets us predict, and predicting lets us change phenomena to our advantage.
Every sensation is true
A sensation only records what affects it; error comes from the judgment we add, and without sensations we can judge nothing.
A true idea is one that can be verified: truth happens to an idea
To call an idea true is to say it guides us well: we can verify it, confirm it and rely on it without being let down.
We must doubt
Our reasons for belief are weaker than we think: certainty escapes us.
9 argumentsThe fetishism of commodities
By exchanging products, people link their labor without knowing it, and this looks to them like a property of things.
Knowing that you do not know: the unexamined life
Nobody truly knows what matters most; the only human wisdom is not to think one knows, which self-examination allows.
There are no facts, only interpretations
To know is always to see from a point of view; a gaze with no needs or desires that sees the thing in itself makes no sense.
Cotton and fire: the cause is not necessary
We see the fire, then the burning, never a link forcing one to follow the other; denying that link is not contradictory.
The problem of induction
Predicting the future assumes it will resemble the past, which can be neither proven by logic nor by experience without going in circles.
How can anyone be Persian?
We judge everything by looking back at ourselves: what seems normal or odd only shows who we are.
By convention sweet, in reality atoms and the void
What we sense changes with the state of our body, so it cannot tell what a thing is: in reality there are only atoms and void.
Agrippa's trilemma: every justification collapses
Every proof needs another, and the chain ends only in regress, circle or assumption, none of which proves anything.
What do I know? Neither senses nor reason can judge themselves
The senses give only appearances, and reason can only be checked by another reason, endlessly.
What escapes words
The most important truths are lived, felt or shown rather than proved.
10 argumentsKnowing what you know, knowing what you do not know
Knowing means telling what you know from what you don't; we do not yet know life, so we cannot speak of death.
The Dao that can be named is not the true Dao
The Dao comes before all distinctions, and words only serve to distinguish: they cannot grasp it as it is.
Reason is a correct scale, but mountains cannot be weighed on it
Our ideas come from what the senses perceive, so reason cannot establish anything certain about what lies beyond the senses.
Whereof one cannot speak, thereof one must be silent
A sentence makes sense only if it states a possible fact, and value and the meaning of the world are not facts.
The meaning of a word is its use
Many words have meaning without naming an object or sharing a common essence: their meaning is how they are used.
Doubt and the light
If reason can be doubted, no proof can rescue it; certainty that returns without proof must be a gift from God.
Why is there something rather than nothing?
In anxiety, nothingness shows itself before any negation: the "no" rests on it, not the reverse, so the question keeps its sense.
Subjectivity is truth
Objective inquiry never ends, but the decision about my life is urgent: essential truth lies in my passionate relation to it.
The two truths: saying the ultimate with words that are not it
Without the words and conventions of ordinary life, nothing can be taught about ultimate truth.
The poisoned arrow
A wounded man who demands answers before being treated dies before getting them: some questions do not lead to liberation.
Other philosophers' positions arrive as profiles are completed. No position is presented as the winner.