The fallacy is when someone demands either an impossible source or their criteria for accepting a source as believable is impossible to meet.


"Driving stoned is dangerous and you should not do it."

"Unless you can can produce a peer reviewed, double blind study that conclusively proves it to be unsafe then it must be safe."

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  • I don't see the demand as fallacious but as merely unreasonable. – Geoffrey Thomas Jan 17 '18 at 16:55
  • Could be a bias - e.g. I require more evidence on conclusion I don't like and less evidence on conclusions I expect. – Vlastimil Ovčáčík Nov 10 '18 at 13:18

In the given example, it's not the demand for robust proof that is unreasonable, but what is inferred from the lack of its fulfillment. The idea that the lack of a robust study proves the opposite is an argument from ignorance.

The number of sources has no bearing on logical inference (cf. appeal to popularity). A single reference, if its premises are accepted, it's data reliable and unbiased, it makes a valid argument and its conclusion is sound, is sufficient. Conversely, a million references that each fail to prove the conclusion individually, may still fail to prove it as a group (though that number may ultimately wear down and convince any human). However, if a number of studies provide different data points, a meta-study can infer new conclusions. Take care here to distinguish logical inference from statistical significance.

While robust sources are desirable, they're not always available. Sometimes the studies just haven't been done yet, or it may not be possible (for physical, practical, ethical or other reasons) to achieve the level of robustness desired. That doesn't invalidate any argument or prove the opposite, it just leaves the audience with more space for doubt. An imperfect study can nevertheless be convincing if it can be ascertained that any flaws or biases were insufficient to skew the conclusion.

In some cases, absence of evidence can be taken as evidence of absence, for example when the claim is dependent on or predicts an effect that should be observable. However, we need to be careful, and like with positive claims, demand a valid argument to ensure the conclusion follows from what has (or hasn't) been observed.

  • Isn't the burden of proof on the claimant for "driving stoned is dangerous"? It seems to me to be fair to ask for proof and assume that such a condition while driving is safe otherwise (from a logical standpoint). If I were to make the same claim about red lollipops (driving with a red lollipop is dangerous), you would be completely justified to ask for my proof, and also be justified in assuming it is safe to drive as such. – TheHonestAtheist Mar 16 '18 at 16:22
  • Assuming it's safe to drive with a red lollipop can be somewhat justified, since there's no clear connection between a red lollipop and driving (unless you're diabetic or the lollipop falls and rolls under the break pedal). However, there's clear grounds for concern when combining a substance that alters cognition with a task that depends on cognitive abilities for safe execution. I think your assumption is unjustified, but I'm not claiming it's dangerous. The default position is to admit uncertainty, not to make assumptions. – reaanb Mar 16 '18 at 17:51

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