Questions: Burden of Proof and the Presumption Principle

Question 1

A health blogger claims that a herbal supplement cures anxiety. When a skeptic asks for evidence, the blogger replies: 'You can't prove it doesn't work. Millions of people use it.' What is wrong with this response?

• A. The response is valid — if no one has disproved the claim, it is reasonable to accept it provisionally
• B. The blogger is shifting the burden of proof to the skeptic, but the obligation to provide evidence rests on the positive claimant
• C. The response is too emotional to count as a logical argument
• D. Popularity of use is a strong form of empirical evidence and should count in the blogger's favor

Answer: (B)

This is the appeal to ignorance fallacy (argumentum ad ignorantiam): reasoning that because a claim hasn't been disproved, it must be true. The burden of proof falls on whoever makes the positive assertion — that the supplement works. Absence of disproof is not positive evidence. The blogger has made a claim and must supply supporting premises; the skeptic is not obligated to disprove it first.

Question 2

A researcher claims to have discovered a simple plant extract that reverses late-stage Alzheimer's disease. A critic says this claim requires far more evidence than a claim that a plant extract improves mild sleep quality. Why is the critic correct?

• A. The critic is applying a double standard — all claims require the same type and amount of evidence
• B. Because the Alzheimer's claim is more expensive to test, it requires a higher evidentiary bar
• C. The threshold of evidence required scales with how surprising a claim is relative to existing background knowledge
• D. Late-stage disease claims require clinical trials while mild condition claims do not

Answer: (C)

Sagan's principle — 'extraordinary claims require extraordinary evidence' — reflects Bayesian logic: the prior probability of a claim affects how much evidence is needed to update belief in it. A claim that contradicts well-established medical knowledge (that late-stage Alzheimer's can be reversed) carries a very low prior, so evidence must overcome that. The sleep quality claim is modest and consistent with background knowledge. This is not a double standard — it is proportionality, calibrating evidential demands to the probability of the claim.

Question 3

If you can seldom disprove a claim, that constitutes positive evidence that the claim is true.

• T. True
• F. False

Answer: False

This is the appeal to ignorance fallacy. Absence of disproof is not the same as positive evidence. We lack disproof of infinitely many contradictory things simultaneously — if undisprovedeness were evidence, we would be forced to believe them all. The absence of evidence for X is only weak evidence against X when we would expect evidence to exist if X were true. The burden of proof principle holds that positive assertions require positive evidence, not mere absence of counter-evidence.

Question 4

The burden of proof falls on the person making the positive assertion, not on those who are skeptical of it.

• T. True
• F. False

Answer: True

This is the foundational principle of rational discourse. We begin from a baseline of not believing things for which no evidence exists, and we shift belief only when warranted by evidence. This asymmetry is not bias — it is necessary to avoid simultaneously believing infinitely many unverified claims. The positive claimant introduces a new belief candidate; they must supply the premises that justify its adoption. Skeptics are not obligated to disprove claims — that reverses the entire structure of evidence-based reasoning.

Question 5

Why does absence of disproof not constitute evidence that a claim is true? Use the structure of rational discourse to explain.

Model answer: Absence of disproof tells us nothing about a claim's truth — it only tells us no one has yet found a counter-example or disproving argument. If undisprovedeness were treated as positive evidence, we would be logically committed to believing an infinite number of unverified claims simultaneously (since most claims have never been disproved). Rational discourse begins from the baseline of not believing claims until evidence supports them; the positive claimant must provide premises that justify belief, not wait for others to eliminate all alternatives.

The principle connects to the basic structure of argument: premises provide positive grounds for a conclusion. 'No one has disproved X' provides no premise about X's truth — it only makes a claim about the history of argumentation. The Sagan/Bayesian framing adds precision: prior probability matters. Absence of evidence is weak evidence of absence only when we would expect evidence to have appeared if the claim were true. This threshold varies by claim, which is why extraordinary claims require extraordinary evidence.