QuestionsIs the world real?The real-world hypothesis is the simplest
Is the world real?The world is real

The real-world hypothesis is the simplest

The cat argument; the simplicity argument against solipsism

Bertrand Russell 1872 · 1970Clarify thought with logic, doubt dogmas.
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What it claims

Our instinctive belief in an outer world independent of our perceptions need not be given up: we may admit it, with a slight doubt coming from dreams.

Why

A world of objects that exist even when unseen explains our experiences far more simply than an endless dream.

In short

"The real-world hypothesis is the simplest" is an argument by Bertrand Russell (The Problems of Philosophy, 1912, ch. II "The Existence of Matter") on the question "Is the world real?" (answer: the world is real). Its claim: Our instinctive belief in an outer world independent of our perceptions need not be given up: we may admit it, with a slight doubt coming from dreams. According to AskPhilosophy's logic check (Z3 solver), the argument is valid only if one grants a premise the author leaves unstated (P3). AskPhilosophy presents 3 objections, 3 of them with a documented reply, and the original text behind each premise, checked word for word.

The Problems of Philosophy, 1912, ch. II "The Existence of Matter"
Valid with an implicit premise

Can we prove that life is not a long dream, in which everything you see, including other people, is made up by you? Russell answers frankly: no. The idea is not contradictory. But, he says, there is no reason to believe it, and it is a far less simple hypothesis than the common-sense one. His example: a cat appears in one corner of the room, then in another. If the cat exists only when you see it, it springs from nothing each time, and it gets hungry without having eaten. If the cat also exists when you do not see it, everything is explained by what you know about hunger. Russell does not claim a proof: he says our instinctive belief in the world is the best explanation, and we have no reason to give it up. He keeps only "a slight doubt" coming from dreams.

P1There is no logical impossibility in supposing that the whole of life is a dream in which we ourselves create all the objects. So it cannot be strictly proved that anything exists besides ourselves and our experiences.
P2But the hypothesis that there are real objects, which keep existing when we do not see them, accounts for the facts of our experience far more simply. The cat that gets hungry between meals is understood if it exists in our absence; if it is only a set of patches of colour, its behaviour becomes inexplicable.
P3 · implicit premiseBetween two hypotheses that account for the same experience, it is reasonable to keep the simpler one, and there is no reason to reject an instinctive belief unless it clashes with others.
Therefore our instinctive belief in an outer world independent of our perceptions need not be given up: we may admit it, with a slight doubt coming from dreams.

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 P3, the computer finds a case where the dream is possible, the real world is simpler, and yet we do not keep the belief in the real world: nothing says simplicity must guide the choice.
The argument is valid with the hidden premise P3. If the dream is possible (P1), if the real world is simpler (P2), and if we keep the simpler hypothesis (P3), then we keep the belief in the real world. The conclusion is deliberately modest: Russell does not say "proved", but "reasonable". The objections target P2 (is simplicity a guide to truth?) and whether reason can justify it at all.
Analogy. You come home and the hall light is off, though you left it on. You can imagine a thousand strange stories. But you keep just one: someone switched it off. Not because the others are impossible, but because that one explains everything with the fewest things to suppose.

Objections

3

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

George Berkeley (matter is useless) 1710 · attacks premise 2

Even if there were external bodies, we could never know it; and if there were not, we would have the very same reasons to think there were. Supposing bodies does not help explain how our ideas are produced: it adds, without reason, useless beings. The ideas of sense follow fixed laws because a mind produces them in us.

a defender replies
see the argument
Bertrand Russell (the chapter on idealism) defender of the argument · 1912 · Russell's reply
In chapter IV, Russell grants Berkeley that sense-data depend on us. But he rejects what follows: from "being known" one cannot conclude "being in a mind". Berkeley confuses the act of apprehending, which is mental, with the thing apprehended, which has no reason to be.
Where the debate stands (our summary)Our summary: both camps explain the same experiences. Berkeley says his version is more economical (no matter); Russell says it rests on a confusion. The debate over what is "simplest" remains open.
David Hume (reason cannot reach objects) 1748 · attacks premise 2

Whether our perceptions are produced by external objects is a question of fact, which only experience could settle. But the mind has never anything present to it but the perceptions: it can have no experience of their connection with objects. Supposing this connection has no foundation in reasoning.

a defender replies
Bertrand Russell (instinctive belief) defender of the argument · 1912 · Russell's reply
Russell partly agrees: we do not come to believe in the outer world by argument, it is an instinctive belief. He draws a different conclusion: we have reason to reject a belief only if it clashes with another, and this one simplifies and organises our experience without causing difficulty.
David Hume (scepticism persists) 1748
Hume would answer that instinct and reason do not agree: relying on instinct, we believe the sensible image is the object itself; relying on reason, we have no proof that perceptions are linked to objects. Either way, the opinion of an outer world is not rationally established.
The debate goes on (our summary) Our summary: Hume writes before Russell and does not answer him; this counter-reply is a reading of his text (EHU XII). Russell himself keeps "a slight doubt" and finds his argument "less strong than we could wish".
Where the debate stands (our summary)Our summary: Russell and Hume agree that reason alone does not demonstrate the outer world. They differ on the conclusion: for Russell, an ordered and coherent instinct is enough; for Hume, the sceptic keeps the advantage.
René Descartes (the evil demon) 1641 · attacks the conclusion

Descartes supposes that some malignant and powerful demon uses all his skill to deceive him: the sky, the air, the earth and all external things would be only the illusions of his dreams. If this hypothesis is possible, no simplicity guarantees that the world is real.

a defender replies
Bertrand Russell (no certainty, but a reason) defender of the argument · 1912 · Russell's reply
Russell recognises that the demon is improbable but possible, so doubt is permitted. He does not claim certainty: all our beliefs may be mistaken, and all should be held with a slight doubt. But a belief can be rejected only on the basis of another; with no belief to doubt from, nothing is left.
Where the debate stands (our summary)Our summary: Russell and Descartes agree that doubt is logically possible. The disagreement is over what to do next: Descartes seeks certainty, Russell is content with the best explanation.