The alternative fringe has embraced a trendy catchphrase: "Absence of evidence is not evidence of absence." It's cute, it sounds impressively philosophical, and, technically, it actually is true. But it can be deceptive, misinterpreted, and misused. The alternative fringe, which interprets lack of evidence as positive support, in effect expands the slogan to mean: "Absence of evidence is evidence of presence." And the peddler of homeopathic nostrums (water imagined to contain a healing "magnetic resonance" of substances that were diluted in it) has it thus: "Absence of presence is evidence of evidence."
But, as a practical matter, is it really true that absence of evidence is not evidence of absence? And is the slogan useful in critical scientific thinking?
The slogan simply reminds us that we cannot prove a negative, that we can never be 100% sure of anything. If someone claims that a heavy object that we are unable to move would rise up in the air if dropped, we can't prove him wrong, although there is no evidence to support his claim. Although every dropped object in history has fallen down, not up, we can't be absolutely sure the next one won't rise up; we can't prove that it won't. Similarly, if someone says the moon is made of green cheese, maybe it is, although there is no evidence to support the claim; we can't prove that it isn't. Hence, we must concede, absence of evidence is not evidence of absence.
The alternative fringe adores this statement because in an appealing way it says that we can't disprove any of their alternative quackeries; we can't prove they don't work, we can't prove a negative. Maybe the acupuncturist can really find and treat chi, although there is no evidence chi exists; possibly the chiropractor, despite the lack of evidence, can really "adjust" the spine to cure diabetes; maybe homeopathic nostrums do work, despite the absence of evidence.
But aside from reminding us that we are fallible, the slogan's utility as a tool of critical scientific thinking is about zero. We know quite well that if we drop something it will go down, not up; we know the moon is not made of green cheese; we know that homeopathic nostrums (a.k.a. water) are medically worthless. I would strongly advise New-Agers inspired by this motto not to test gravity by jumping off a roof; I would advise any diabetic alternative medicine enthusiast to use insulin, not a chiropractor.
To make this motto useful I suggest the following revision: "While absence of evidence is not absolute evidence of absence, it is generally evidence of a high probability of absence."
Dr. Marvin J. Schissel is a dentist and an advisor to the American Council on Science and Health, the National Council Against Health Fraud, and the Committee for Scientific Investigation of Claims of the Paranormal.
