Fishing For Evidence

Image by Google Maps

Early one morning six boys came across a beautiful stream. None of them had ever tried to catch fish in this stream. Greg, one of the boys, casually looked at the stream and said, "I don't think there are any fish in there". The other boys asked, "Why not? We can't see the bottom of the stream so we can't say there are no fish". They then added, "The absence of evidence is not evidence of absence".

They decided to set up camp near the stream with the intention of spending a few days fishing and camping. They eagerly assembled their fishing tackle and set to the serious business of challenging who would catch the first or the largest or the most fish. The boys each had a preferred technique, some used live bait, some used artificial lures and some had fly fishing gear.

By lunchtime, the boys gathered at their camp. Nobody had caught a fish and nobody even reported a nibble. The mood was starting to change about catching fish in this stream. Some boys even thought Greg was right to declare there were no fish here. A short discussion about the absence of evidence ensued. One boy gave an analogy to explain the idea. He said, "Astronomers know a lot about the planet Mars. They have looked at it through the most powerful telescopes available and are unable to detect any signs of life on Mars. Being unable to detect life at such vast distance is not evidence for the absence of life on Mars."

They all agreed that they would stay and continue fishing. By sunset, still no fish had been caught. As they ate some of the tinned food they brought, the discussion turned again to the absence of evidence. Some asked, "How much absence of fish would it take to declare that there were no fish in this stream?” As the mood, along with the daylight, got darker and darker, it was decided that if no fish had been caught by lunchtime tomorrow they would pack up and find another site.

As agreed the night before, at lunchtime the boys had moved camp and declared that the absence of any evidence of fish is evidence of the absence of fish.

At the new site, which was similar to the old site, Greg mumbled something about how there probably won't be any fish here either. Everyone else groaned. Within half an hour of the first line being cast, fish were caught and Greg was happy to be proven wrong. They enjoyed their fish supper and bragged about the first to catch a fish, catching the biggest and catching the most fish and as all fishermen do, the ones that got away. Later that evening the discussion reverted to the phrase "the absence of evidence".

Greg admitted that he had no way of knowing whether there were fish at either site. He agreed that his absence of evidence was not evidence of the absence of fish. Later another boy reminded them that when they left the first site, they had all agreed that the absence of evidence was evidence of the absence of fish.  They realized, after a day and a half trying to catch fish, that by not catching any, this was evidence that there were no fish to catch.

So how can two such obviously contradictory statements as, "The absence of evidence is not evidence of absence" and "The absence of evidence is evidence of absence" both be correct? The contradiction is due to equivocation (ambiguity) over the meaning of "absence of evidence". Greg had no evidence to support his claim that there were no fish, so his absence of evidence was not evidence of absence (no fish to be caught). After spending a day and a half fishing, the boys now had some evidence that there were no fish to be caught, so the absence of evidence (no fish caught) was evidence of absence (no fish to be caught).

When anyone makes the statement, "The absence of evidence is not evidence of absence" consider carefully what they mean by "the absence of evidence". It may be necessary to ask if evidence has been sought and if it has, then the absence of supporting evidence is indeed evidence against whatever is claimed to exist.