It is a notorious fallacy to think that the lack of evidence for some notion is evidence for some competing notion. This has been recognized since antiquity that this is a fallacy. It goes by many names, including argument from no evidence, the false dilemma, and argumentum ad ignorantiam (literally, “argument from ignorance”).
Here are some illustrative examples. The first two are slightly contrived, but only slightly so, and the others are distressingly common.
| One morning Salvatio caught a Theba pisana and later that day he caught an Isurus oxyrinchus. His neighbor Simplicio heard about this, and concluded that both must have been caught using the same technique. He argued that the process of catching things was covered by the three laws of motion, which must apply equally in both cases. And since Simplicio didn’t see any reason why the two cases should be different, he decided they were the same. | Simplicio was, alas, unaware that Theba pisana is a garden snail, while Isurus oxyrinchus is a shark. Salvatio caught one with his bare hands as it slithered across the walkway, and he caught the other using a big hook and heavy tackle. It’s true that Simplicio didn’t know why the two cases were different, but that’s just because he had no clue about what was going on in either case. |
The fallacy is equally fallacious in the other direction:
| One morning Salvatio caught a Hyla crucifer and later that day he caught a spring peeper (a small frog). His neighbor Simplicio heard about this, and concluded that they must have been caught using completely different techniques. He argued that he had no reason to believe the two things were the same, so they must be different. “I don’t believe in coincidences”, he said. | Well, it turns out that a Hyla crucifer is nothing more or less than a spring peeper. Simplicio had no reason to believe the two were the same, but that’s just because he had no clue about what was going on in either case. |
| It is all-too-common to hear people claim that quantum mechanics explains the origin of consciousness. They argue that QM seems mysterious to most people, and consciousness also seems mysterious ... therefore they must be the same, or closely related. | This is based on nothing more than the fact that they don’t understand quantum mechanics and don’t understand how the mind works, either. If you look closely enough, QM applies to everything, but so what? QM has at least as much to say about a chunk of iron as about a brain ... but that doesn’t mean that iron is conscious. |
| It just cracks me up when people invoke the Coanda effect to “explain” how ordinary wings produce lift. It is said that the Coanda effect is governed by the three laws of motion, and ordinary wings are, too, so the two must be the same. | You can be pretty sure that anybody who says this doesn’t know what the Coanda effect is, and probably doesn’t know how wings work, either. This is just the snails-and-sharks argument again. Lots of things are governed by the same fundamental laws yet have little else in common. |
| It is all-too-common to hear people arguing in favor of “intelligent design”. Simplicio says “I don’t understand how evolution could have produced something as elegant and wonderful as ...., and therefore it must have been designed by God”. | Well, the first part is true: Simplicio doesn’t understand how evolution could have produced the structures in question. But that’s just because he doesn’t understand. His lack of understanding is not evidence for (or against) anything. |
The “intelligent design” faction uses many fallacies (not just argument from no evidence), as discussed in reference 1.