Claim a fallacy and win the argument?

The internet sure has opened up space for people to argue. We’ve got plenty of quantity but not always so much quality!

Often an argument is met with the claim that the proponent has committed a fallacy. Sometimes the accusation is true, but not always.

Here are three alleged fallacies I have come across recently. I wonder how you think they stand up?

Argument from authority

A logical argument is supposed to be based on evidence, and it is up to the proponent to provide sufficient evidence to convince a reasonable person. If instead of providing evidence, they justify their premises by quoting somebody’s opinion, this is deemed an argument from authority, and a classic logical fallacy.

But difficulties arise if the evidence is specialised, for example a complex area of science, or the assessment of a historical matter. These are not areas where many of us have expertise and therefore don’t have the first hand information to present as evidence. We must rely on experts. That is why scientific and other academic papers almost always quote many references – so that they can establish the bona fides of the information they present.

But quoting an authority in support of a proposition requires two conditions:

  1. The person quoted has to be a real expert in the matter being discussed, not just someone claiming expertise, or a celebrity, or someone whose expertise is in another area of learning.
  2. If the experts don’t all agree, just quoting one expert (which is a common ploy of people with more extreme views) is likely to be insufficient to establish a fact – we need to quote the consensus of experts.

Thus we find that the Stanford Encyclopedia of Philosophy says of this fallacy: “Fundamentally, the fallacy involves accepting as evidence for a proposition the pronouncement of someone who is taken to be an authority but is not really an authority. Similarly, when there is controversy, and authorities are divided, it is an error to base one’s view on the authority of just some of them.”

So many objections to the use of the conclusions of experts, claiming this fallacy, are in fact quite incorrect. It is generally helpful to quote experts provided we do it honestly.

The Prosecutor’s Fallacy

I had never heard of this fallacy until recently, when someone claimed the fine-tuning theistic argument is based on it. It turned out they didn’t understand the fallacy themselves, but it did lead me to check it out.

This is a fallacy in the use of statistics, specifically conditional probability, which is the probability of an event occurring, given that some other condition has been met. An example might be the probability of a person dying from lung cancer granted that they are a regular smoker.

The fallacy occurs when someone confuses or swaps the probability and the condition. For example, the probability that a smoker will die of lung cancer is assumed to be the same as the probability that a person who died from lung cancer was a smoker.

You may think this is an obscure and theoretical issue, but it has been important in a number of high-profile legal cases. This lecture on the fallacy gives an interesting example where accused IRA bombers (the “Birmingham Six”) were convicted partly on the basis of this fallacy. The Six were tested for having handled explosives and two were found to be positive, using a test that gave a positive result with 99% accuracy if a person had handled them (probability of test result if explosives). However the prosecution, made the claim that this test proved with 99% certainty that they had handled explosives (probability handled explosives if positive result), ignoring the fact that the test would also give a positive result for handling a number of other chemicals found in everyday life. The probability these men had handled explosives was significantly less that 99%!

It turns out that, as far as I can tell, the fine tuning argument doesn’t commit the Prosecutor’s Fallacy, but it is something to watch out for in arguing from probability.

God of the gaps

God of the gaps reasoning occurs when a scientific unknown (a gap) is used to argue for the existence of God. Atheists criticise God of the Gaps because science is slowing filling the gaps, making God unnecessary, they say. So, it is alleged, God of the gaps is a fallacious way to argue.

I have looked at God of the Gaps in some detail, and I’m not so sure it is a bad type of argument, at least not always. I won’t go through all the details here, but I came to these conclusions:

  1. Metaphysical reasoning cannot totally avoid gaps in human knowledge, but the best reasoning is built not on gaps in knowledge but on the best scientific understanding available.
  2. New developments in science can lead to new scientific understandings and new religious or philosophical understandings. This shouldn’t cause religious believers any difficulty.
  3. An event can reasonably be described from different perspectives – science may explain the processes, but personal choices or the action of God may explain something different but important.
  4. There are some aspects of life and the universe where science reaches its limits. We can reasonably ask if theism explains what science cannot.
  5. There are thus several ways that science may form the basis of a theistic argument. In all cases we can consider any accepted scientific conclusions, and ask the simple question – are these facts more likely if naturalism is true, or more likely if theism is true?
  6. It is better to explain why we believe someone’s reasoning is faulty rather than thinking that labelling it “God of the gaps” is sufficient to discredit it.

So God of the gaps reasoning may sometimes be a faulty argument, but it is still a matter of the proponent offering justification for their premises, and objectors attempting to show the premises are unjustified. Just saying “God of the gaps!” is hardly a refutation of anything.

So what?

There are many logical fallacies, but we need to be sure we understand them well before we accuse someone else of using them, and therefore being wrong.

Graphic: Gallery4Share


  1. Haha! Not quite that bad. But I did recently get someone arguing that the fine tuning theistic argument committed the Prosecutor’s Fallacy, but when I asked them what it was, they gave a completely wrong explanation.

  2. I’d believe that. I’m not even sure what mixing up conditional probabilities would achieve in the context of the fine-tuning argument.

  3. When this person mentioned it, I hadn’t heard of it. I looked it up and found it difficult at first to understand the explanation (e.g. in Wikipedia), until I found a short video from the University of Zurich which was very clear. But it led me to think that this person didn’t understand it either, but assumed something, but got it totally wrong. Even after we discussed it at length, I still don’t think he understood it. It was a classic case of Dunning Kruger.

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