QuestionsShould we believe without proof?Extraordinary claims require extraordinary evidence
Should we believe without proof?No belief without evidence

Extraordinary claims require extraordinary evidence

The Sagan standard; ECREE

Carl Sagan 1934 · 1996Astronomer and science communicator who popularised the demand for solid evidence for extraordinary claims.
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What it claims

As long as extraordinary evidence is missing, we have no reason to believe an extraordinary claim; we can study it without believing it.

Why

The further a claim departs from what we know, the stronger the evidence needed, and whoever makes the claim must supply it.

The same idea through history
David Hume1748Hume lays down the core principle: the wise man proportions his belief to the evidence, and no testimony suffices to establish a miracle unless its falsehood would be more miraculous than the fact.William K. Clifford1877Clifford makes it a moral rule: it is wrong to believe on insufficient evidence.
Carl Sagan1979In Broca's Brain, about pseudoscience, Sagan writes that the extraordinary should be pursued, but that extraordinary claims require extraordinary evidence. He takes up the phrase in the series Cosmos (1980).
Pierre-Simon de Laplace1814"The more extraordinary a fact, the more it needs to be supported by strong proofs": the ancestor of Sagan's phrase. Laplace gives the reason: those who attest the fact may deceive or be deceived, and both causes are the more probable the less probable the fact is in itself.
In short

"Extraordinary claims require extraordinary evidence" is an argument by Carl Sagan (Broca's Brain, 1979; Cosmos, 1980, 1979) on the question "Should we believe without proof?" (answer: no belief without evidence). Its claim: As long as extraordinary evidence is missing, we have no reason to believe an extraordinary claim; we can study it without believing it. 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, 1 of them with a documented reply, and the original text behind each premise, checked word for word.

Broca's Brain, 1979; Cosmos, 1980
Valid with an implicit premise

In 1979, in Broca's Brain, the astronomer Carl Sagan reports a debate within an American committee of sceptics: should all doctrines that smell of pseudoscience be combated, or should each issue be judged on its merits, with the burden of proof on those who make the proposals? He sides with the second camp. The extraordinary should be pursued, he writes, but extraordinary claims require extraordinary evidence. The phrase, taken up in the series Cosmos (1980), extends an older principle, Hume's on miracles. It applies to any out-of-the-ordinary claim, religious ones included. A caveat: Sagan does not offer it as a demonstrated argument but as a rule of method; we re-read only the passage of Broca's Brain, not the text of Cosmos.

P1Each issue should be judged on its merits, but the burden of proof falls squarely on those who make the proposals.
P2The extraordinary should certainly be pursued, not dismissed in advance.
P3 · implicit premiseA claim that contradicts much of what is firmly established is, before examination, less likely: the wise man proportions his belief to the evidence, so he needs proportionally stronger proof.
Therefore, as long as extraordinary evidence is missing, we have no reason to believe an extraordinary claim; we can study it without believing it.

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 burden of proof falls on the claimant and the extraordinary is examined, but where ordinary evidence would suffice to believe an out-of-the-ordinary claim.
Valid with P3. Without P3, nothing says the amount of evidence required grows with the strangeness of the claim; one could ask the same evidence for all. P3 is the premise in debate: who decides what is "extraordinary", and relative to which background knowledge? And to what extent does every belief need evidence?
Analogy. If your neighbour says he bought a cat, you believe him. If he says there is a dragon in his garage, you want to see the dragon, not only hear about it.

Objections

3

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

M
Marcello Truzzi 1987 · attacks premise 1

Truzzi, a sociologist and sceptic, accepts the rule: the more extraordinary a claim, the heavier the burden of proof. But he distinguishes the true sceptic, who only says "it is not proved", from the "pseudo-sceptic", who denies. Whoever asserts that a strange result comes from an error or from cheating also puts forward a thesis, and so must prove it in turn.

no reply
We have not yet found a direct reply by Sagan in the texts.
The debate goes on (our summary) Sagan writes that the extraordinary should be pursued (P2) and each issue judged on its merits: this does not answer Truzzi directly, but shows that Sagan does not ask to deny without examination.
Where the debate stands (our summary)Truzzi and Sagan agree on the rule; they differ on its use. For Truzzi the burden also weighs on whoever puts forward a counter-explanation; Sagan speaks only of the one who proposes. Both hold that denying without investigation is a fault.
Alvin Plantinga 2000 · attacks premise 3

Demanding evidence for every belief is a demand we do not keep for other beliefs (memory, other minds). Some beliefs are "basic": starting points for thought, rational without proof. Belief in God may be one of them, and Sagan's standard does not apply to it.

a defender replies
see the argument
William Kingdon Clifford defender of the argument · 1877
Clifford holds the opposite rule: it is wrong, always, everywhere and for anyone, to believe anything on insufficient evidence. This text predates Plantinga by a century; it does not answer him directly.
see the argument
William James 1896
James answers Clifford that our passional nature may, and must, decide, when the option is genuine and cannot be decided on intellectual grounds. Clifford's rule would not hold for such cases.
see the argument
Where the debate stands (our summary)The core disagreement: must every belief be supported by evidence, or may some serve as starting points? It has opposed Clifford to James and Plantinga for over a century, and is unsettled (our summary).
R
Richard Price 1767 · attacks premise 3

Against Hume, Price notes that a particular lottery draw is improbable beyond all conception: with 50,000 tickets, each order of drawing has almost no chance. Yet the testimony of a newspaper or of any ordinary person is enough to make us believe it. A very improbable claim therefore does not always require out-of-the-ordinary evidence.

no reply
No reply possible: objection raised in 1767, after the author's death (1996).
The debate goes on (our summary) Price writes two centuries before Sagan, against Hume. A supporter of Sagan answers that a particular draw is improbable but not extraordinary: some draw must happen, and it contradicts nothing we know. A supporter of Price answers that one must then say what makes a claim "extraordinary", and that the standard does not say it.
Where the debate stands (our summary)Open point: Sagan's standard does not define "extraordinary". Its supporters mean what contradicts established knowledge, not the merely improbable; its critics see a risk of conservatism. No precise rule has consensus.