Flawctober: The Absence of Evidence Fallacy

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In preparation for the November 2018 LSAT, we’re going to spend the month of October going over some of the most common logical fallacies we see on the LSAT. Being able to reliably identify these fallacies will help you out on a ton of questions on the LSAT — about half the questions in Logical Reasoning section, in fact. Over this month, we’ll be doing such a thorough investigation into some of these common fallacies that we hereby dub this month Flawctober. Today, we’re discussing the absence of evidence fallacy.

“Professor Jones believes the ark is located in Egypt. However, the sources he trusted were recently shown to be lying, so the ark must be somewhere other than Egypt.”

“I saw a ghost in my house last night. My friend Shaggy told me that no one has ever proven the existence of ghosts, so what I saw must have been my friend wearing a sheet.”

That’s right, today we’re discussing the spookiest of fallacies – the absence of evidence fallacy. You can’t prove that vampires don’t exist; therefore, they must exist! OoOoOoO so spooky!

What is the “absence of evidence” fallacy?

The absence of evidence can occur a few ways. Someone could say that, because there is no evidence for a position, that position must be false. Or, someone could say that because the evidence against a position was recently disproven, the position must be correct after all. You get the idea.

In other words, the absence of evidence fallacy occurs when someone uses a lack of evidence to try to “prove” something. Of course, the problem with this line of reasoning is that a lack of evidence is just that: a lack. You can’t use it to conclude anything; you could only conclude that we still don’t know about that thing.

How can I identify when an argument contains this fallacy?

There are a couple features to keep an eye out for that might help you identify arguments containing absence of evidence fallacies.

For one thing, they’ll typically include a sharp about-face: the premises will be about one thing, and then suddenly the argument will say “well, we can’t prove that thing, so it must be the opposite!”

For another, the conclusion is typically very strong — the argument will conclude that something must be true or false (instead of the more nuanced, and less flawed, conclusion that something might be true, or that we don’t know whether that thing is true).

But it really comes down to the support given in the premises: Do the premises cite some sort of evidence, and then say the evidence has been disproven (as in the example about the mysterious Professor Jones above)? Do the premises say that something has never been affirmatively proven or disproven (as in the ghost example above)? And then does the conclusion say that as a result, the opposite must be true? If so, yep, that’s an absence of evidence fallacy.

How would I strengthen or weaken an argument containing this fallacy?

Really, the easiest way to fix an argument that has an absence of evidence fallacy is to change the conclusion — in the examples above, we’d like the argument to conclude that the ark might not be in Egypt or that I may not have seen a ghost.

But unfortunately, the LSAT will never give you an option to straight-up change a conclusion; you’ve got to work with what you’ve got. So for this type of flaw, you’d need to change the support to make it more or less appropriate.

For instance, if you added the information that “it is very likely that if ghosts were real, their existence would have been proven by now,” the argument becomes at least somewhat more sensical. Or, if you added the information that “although the sources saying the ark was in Egypt were wrong, other people have also claimed the ark is in Egypt,” suddenly the conclusion makes even less sense. Effectively, you’re using the premises to either somewhat fix the flawed line of reasoning, or to call even more attention to that flaw.

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