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The Tree of Knowledge

I Don’t Believe It!

Dene Bebbington presents a couple of bad but popular arguments.

How often have you read or heard someone say something like “That’s not possible, it couldn’t have happened like that”? That person may think it’s a clever rebuttal which stops debate in its tracks and gives them the rhetorical victory, but it’s a fallacy in informal logic known as the argument from personal incredulity.

Humans have a need for certainty – we have a psychological preference against doubt – which is why this fallacy has a particular allure. To understand what’s going on when someone has committed this fallacy, we can unpack its unstated logic:

1. I can’t imagine how this thing could be true

2. But if this thing is true then I should be able to imagine how it’s true

3. Therefore this thing is not true

Once the premises of the argument, 1 and 2, are laid bare, its flaw is exposed. The premise that ‘If this thing is true then I should be able to imagine how it’s true’ is evidently false, or at least, is not necessarily true.

A form of this fallacy crops up for instance in the debate about Intelligent Design, which posits that life on Earth is not the result of purely undirected evolutionary processes but includes the influence of intentional design, working through those same evolutionary processes. For instance, Michael Behe, Professor of Biochemistry at Lehigh University in Pennsylvania, claimed in his book Darwin’s Black Box (1996) that a tiny bacterial flagellum couldn’t be explained by undirected evolution because it is irreducibly complex. He wrote, “Since natural selection can only choose systems that are already working, then if a biological system cannot be produced gradually it would have to arise as an integrated unit, in one fell swoop, for natural selection to have anything to act on”(p.39). This argument might be said to be based on Behe being unable to imagine how such a complexly integrated biological system could naturally evolve. The analogy he used was a mousetrap, with spring, catch, hammer, hold-down bar and platform. He said this is irreducibly complex because omitting any component means it cannot work at all. All five parts working together were needed to catch any mice. Yet once he published this, several people did show how the mousetrap would function with fewer components. It has been countered that the concept of irreducible complexity used in intelligent design arguments is not an argument from personal incredulity, but rather an argument arising out of the fact that the community of biological science has failed to demonstrate how the flagellum and other irreducibly complex biology could have arisen. Meanwhile, an argument from the other side, against an Intelligent Designer, is that there’s no known way the Designer could manipulate matter. However, this can also be construed as an argument from personal incredulity. Though we can’t imagine of how a Designer could create a flagellum, or work through evolution in other ways, that may just be due to a limitation of our current knowledge.

fish 1
Just because you can&rsquo't imagine how it could happen…
Fish images by Paul Gregory

At the root of the disagreement is the thorny problem of who bears the burden of proof. For the discussion to progress, people on opposite sides of the debate would need to agree on who has the burden of proof, and what would constitute sufficient evidence for a proposition to be at least provisionally accepted as scientific. The failure of participants in the intelligent design versus undirected evolution debate to reach an agreement here means that the issue is unlikely to be settled in the foreseeable future.

fish 2
…doesn’t mean it didn’t happen


A related informal fallacy is the appeal to ignorance (Latin, argumentum ad ignorantiam). This also underpins some arguments in support of Intelligent Design, which boil down to saying that undirected evolution has not been proven to be true in certain cases, therefore it’s false. The fallacy also works the other way round, in a form which has been used by Intelligent Design opponents: Intelligent Design has not been proven to be true, therefore it’s false.

Classic examples occur in the perennial debates over the existence of God. On one side of the debate, an atheist might say that no proof of God’s existence has been given, therefore God doesn’t exist. Contrarily, a theist might claim that since God’s existence hasn’t been disproved, then God does exist – with the assumption that the prevailing arguments for God’s existence are considered sufficient to persuade.

The fallacy has two, converse, forms:

This hasn’t been proved to be true, therefore it’s false

This hasn’t been proved to be false, therefore it’s true

Absence of evidence isn’t logically equivalent to evidence of absence, but to believe it is can be considered to be an application of the first form of the fallacy.

The second form of the fallacy is a weaker type of reasoning than the first because proving a negative is generally more difficult than proving a positive. It has been claimed by some theists that because atheists cannot disprove the existence of God, then the default position should be that God exists. This default could be further defended on the grounds that religious belief has been endemic throughout history, or because the physical constants of the universe are fine-tuned, for example. Compare this with the New Atheist idea that because theists cannot prove the existence of God, the default position should be that God does not exist.

For a debate to be productive there should first be agreement as to which side bears the burden of proof. Without such agreement, the argument to ignorance is likely to be used by whichever side believes the onus is on their opponent to prove their case. The fallacy can be hard to eradicate from debate even when its use has been demonstrated by outside parties. Each side may believe they have provided sufficient evidence for their case, and so consider the fallacy’s form of reasoning to be valid. Further exploration of this would involve opening the can of worms of cognitive biases, and why and how people are resistant to evidence or reasoning opposed to their own worldview.

However, in informal logic, the situation is not always black and white. Context determines whether the argument to ignorance is a logical fallacy or a reasonable argument. Consider a more down-to-earth example. In a criminal trial the burden of proof is on the prosecution because in law defendants are to be presumed innocent until proven guilty. The evidential bar is set high as a safeguard to avoid sentencing those who are innocent, albeit with the risk that guilty people escape punishment. It results in judgements that, for example, ‘The prosecution hasn’t proved beyond reasonable doubt that the defendant is guilty, therefore the defendant is not guilty’. So a jury’s decision to find the accused not guilty of a crime might well be fallacious in logic, but legitimate in legal practice. Similarly, scientific theories are supposed to be tentative until falsified, and modified or replaced in light of new evidence. This means that science can provide another reasonable use of the second form of the argument to ignorance: ‘It hasn’t been proved to be false, therefore it’s (provisionally) true’ – provided that there are good grounds for accepting the theory for the time being, even though it hasn’t been proved.

Whilst we should avoid dogmatic reasoning from ignorance, it’s important to recognise that there are strong and weak uses of the fallacy, and that fallacies are not usually employed intentionally, to mislead or deceive. After all, it’s easier to identify fallacies when used by another party of whose position we are critical, than identify them when used by our own party. Moreover, there’s nothing wrong with admitting ignorance. It can be a spur to further learning and improving our reasoning ability. As Socrates said, the beginning of wisdom is admitting one’s ignorance.

© Dene Bebbington 2021

Dene Bebbington is a freelance writer with an interest in informal logic.

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