Absence of Evidence Is Not Evidence of Absence

Carl Sagan? Martin Rees? William Wright? William Housman? W. J. Sollas? Dugald Bell? Anonymous?

Dear Quote Investigator: The existence or non-existence of intelligent extraterrestrial life is a highly contentious subject. Some thinkers who are open to the possibility of interstellar aliens also believe that the current evidence is inadequate; hence, they advocate using radio telescope dishes as listening devices to collect more data. They also point to the following maxim to discourage premature judgments:

Absence of evidence is not evidence of absence.

This saying has been attributed to cosmologist Martin Rees and astronomer Carl Sagan; however, I think it was circulating before these gentlemen were born. Would you please explore this topic?

Quote Investigator: Tracing this statement is difficult because it has evolved over time. A partial match for six of the eight words occurred in a scholarly paper read during a meeting of the “Victoria Institute” held in London in 1887. The Reverend William Wright’s paper titled “The Empire of the Hittites” argued that data about the movements of the Hittite people was incomplete; therefore, this paucity of evidence should not result in firm conclusions. Boldface added to excerpts by QI: 1

It is urged that the Hittites could not have been settled in Southern Palestine because there are few direct references to their southern settlements in the inscriptions. To this I reply, that the absence of evidence is not evidence. The Egyptians marched up the coast of Syria, and turned inland to Megiddo and Kadesh, where they met the Hittites. The inscriptions are full of the doings of the Hittites at Megiddo and Kadesh, because the Egyptians went thither. They have nothing to say of the Hittites of Hebron, because the Egyptians did not go thither. The inscriptions are records of what happened during campaigns in which Egypt must have made great sacrifices.

Below are additional selected citations in chronological order.

In 1891 the “Live Stock Journal” of London published letters between two people who sharply disagreed about the movements and location of a bull. The letter from William Housman contended that there was an important difference between absence of evidence and evidence of absence, i.e., the two were not logically equivalent: 2

Mr. Fowler takes up again the original question of the controversy, and asks me to account for an alleged break of three years in the evidence of Lancaster Comet’s presence at Lenton. If, he says, I cannot do that, he thinks we are obliged to conclude that the old bull left Lenton about March, 1856, and went to Sittyton in 1859. May I point out to Mr. Fowler the distinction between absence of evidence and evidence of absence?

In August 1895 “The Quarterly Journal of the Geological Society of London” published an article by W. J. Sollas who was a Professor of Geology at the University of Dublin. Sollas disagreed with scientists who used the absence of evidence as evidence of absence in the domain of glaciology: 3

Those who treat the absence of evidence as evidence of absence have asserted that by no possible process can stones be introduced from the floor of a glacier into its substance, and it is possible that they might until quite recently have objected that, while ice might ascend South Barrule, it could not pick up fragments from Granite Mountain; fortunately we are now in possession of Mr. Chamberlin’s observations on the glaciers of Greenland, which not only show that the lower layers of the ice are densely charged with detritus, but explain the manner in which it is introduced from the bottom.

Finally, in the December 1895 issue of “The Glacialists’ Magazine” an exact match for the maxim appeared in an article by Thomas Sheppard, but the author credited another scientist: 4

Even if no moraine existed, it does not follow that there was no Ice-sheet : it has been remarked by Mr. Dugald Bell that “absence of evidence is not evidence of absence.”

In 1939 “The Yorkshire Post” of England printed an article containing a closely matching statement: 5

But there is still a gap of a thousand years to be bridged and we venture to suggest that during this period boats containing the dead were burnt, probably at sea, as was sometimes the practice of the later Vikings. The absence of evidence is not necessarily evidence of absence, especially prehistoric matters where so many wooden objects like boats have irrevocably perished.

In 1947 “The Hartford Courant” of Connecticut printed the adage within an article by a naturalist who was hunting for orchids: 6

I wondered if I had searched carefully enough, and I recalled a warning which reads: “Absence of evidence is not evidence of absence.”

In 1953 “The Honolulu Advertiser” of Hawaii printed the saying within an article discussing a species of flightless bird: 7

What the scientists can’t figure out is whether or not the moa still exists. No man now living has ever seen one, and for that reason the bird has been classed as extinct. But “absence of evidence is not evidence of absence”—and there is a million acres of wild, unexplored country in the South island of New Zealand which might very well harbour whole tribes of moas.

In 1971 a summer program sponsored by the National Aeronautics and Space Administration (NASA) formulated and analyzed strategies for detecting extraterrestrial intelligent life. A technical report from the project employed the following quotation as an epigraph for the second chapter: 8

“Absence of evidence is not evidence of absence.”
Martin Rees

In December 1972 the adage was employed by astronomer Richard Berendzen during a symposium discussion about the possibility of life on other worlds: 9

Acknowledging that there is no data whatever on the existence of life anywhere but on earth, Berendzen pointed out at the start that “absence of evidence is not evidence of absence.”

In 1977 astronomer and science popularizer Carl Sagan published “The Dragons of Eden: Speculations on the Evolution of Human Intelligence”. Sagan used the saying while discussing the brain: 10

A few students of the subject seem to have concluded that, because they have been unable to isolate and localize all higher brain functions, no future generation of neuroanatomists will be able to achieve this objective. But absence of evidence is not evidence of absence. The entire recent history of biology shows that we are, to a remarkable degree, the results of the interactions of an extremely complex array of molecules . . .

In conclusion, QI believes that this maxim evolved over time. A truncated version from William Wright appeared in 1887. An instance with different phrasing appeared 1891. An exact match attributed to Dugald Bell appeared in 1895. Future researchers may discover earlier instances. Martin Rees and Carl Sagan employed the saying and helped to popularize it many years after it was in circulation.

Image Notes: Illustration of extraterrestrial robotic lifeforms from KELLEPICS at Pixabay. Image has been cropped and resized.

(Special thanks researcher Stephen Goranson who discussed this topic and located the two citations in 1895. Also, great thanks to @hullodave whose joke and @szescstopni whose inquiry led QI to formulate this question and perform this exploration. In addition, thanks to researcher Barry Popik for his pioneering efforts.)

Notes:

  1. 1888, Journal of the Transactions of The Victoria Institute, or Philosophical Society of Great Britain, Volume 21, Ordinary Meeting on January 3, 1887, (Paper read at the meeting by the author), The Empire of the Hittites by the Rev. William Wright, Start Page 55, Quote Page 59, Published by The Victoria Institute, London. (HathiTrust Full View) link
  2. 1891 October 16, Live Stock Journal, Letter title: The Lenton Herd, Letter from: Wm. Housman, Quote Page 408, Column 1, Vinton and Company, London. (Google Books Full View) link
  3. 1895 August, The Quarterly Journal of the Geological Society of London, Volume 51, An Experiment to Illustrate the Mode of Flow of a Viscous Fluid by W. J. Sollas (Professor of Geology in the University of Dublin) Paper was read April 24th, 1895, Start Page 361, Quote Page 367 and 368, Longmans, Green, and Company, London. (HathiTrust Full View) link
  4. 1895 December, The Glacialists’ Magazine: A Quarterly Record of Glacial Geology, Volume 3, Part 3, On the Occurrence of Scandinavian Boulders in England by Thomas Sheppard, Start Page 129, Quote Page 132, Available from F. H. Butler, London, Printers and Lithographers Jowett & Sowry, Leeds.(HathiTrust Full View) link
  5. 1939 August 14, The Yorkshire Post, Prehistorical Boat-Burial: First-Hand Account of a Great Yorkshire Discovery by Dr. and Mrs. Frank Elgee, Quote Page 4, Column 4, Yorkshire, England. (British Newspaper Archive)
  6. 1947 October 19, The Hartford Courant, Section: The Hartford Courant Magazine, A Naturalist Afield, Quote Page 10, Column 5, Hartford, Connecticut. (Newspapers_com)
  7. 1953 July 19, The Honolulu Advertiser, Section: The Hawaii Weekly, Tip to Fortune Hunters by A. L. Kidson, Quote Page 3, Column 1, Honolulu, Hawaii. (Newspapers_com)
  8. 1972, Project Cyclops: A Design Study of a System for Detecting Extraterrestrial Intelligent Life, Prepared Under Stanford / NASA / Ames Research Center, 1971 Summer Faculty Fellowship Program in Engineering Systems Design, Document CR 114445, Chapter 2: Life in the Universe, (Epigraph of Chapter 2), Quote Page 3, Published by NASA, U.S.A. (Accessed via ntrs.nasa.gov on September 14, 2019) link
  9. 1972 December 6, The Evening Press, When the Stars Speak by William Hines (The Press Chicago Sun-Times), Quote Page 16B, Column 1, Binghamton, New York. (Newspapers_com)
  10. 1977, The Dragons of Eden: Speculations on the Evolution of Human Intelligence by Carl Sagan, Chapter: Introduction, Quote Page 7, Random House, New York. (Verified with scans)