Absence of Evidence: The LSAT Trap

Adrian Calloway

So the, the researcher runs a preliminary trial on this new cardiac medication, right? And the trial comes back, well, inconclusive. It, it failed to prove that the medication actually lowers blood pressure. But then the researcher turns right around and concludes that the medication is definitely completely useless. That it, it simply does not work.

Nora Ashford

Classic. I mean, that is, that is literally the first trap I used to fall for every single time on Section Two.

Adrian Calloway

It is a classic LSAT move, yes. And, and what is happening logically there is what Carl Sagan famously summarized as, you know, absence of evidence is not evidence of absence. Or to put it in formal terms, the lack of evidence for the existence of something does not necessarily prove that it does not exist at all.

Nora Ashford

Right, because, okay, if I say I checked my backpack and I, I did not find my keys, that does not mean my keys were annihilated from the universe. It just means, uh, maybe I did not look in the side pocket. Or maybe I left them on the kitchen counter.

Adrian Calloway

Precisely. But in casual, everyday conversation, we, we collapse that distinction all the time. If a friend says, hey, is there any evidence that the new coffee shop on Fourth Street is open yet, and you say, uh, no, I have not seen any proof, your friend naturally hears, okay, it is closed. We, we use no proof as a proxy for false.

Nora Ashford

Yes! That, that was my exact breakthrough when I was studying! I realized the test writers are constantly playing on our human cognitive bias. In real life, if nobody has proven a claim, we practically treat it as fake news. But on the LSAT, a complete lack of proof just leaves us sitting right at status quo uncertainty. We, we just know nothing new!

Adrian Calloway

Exactly. Status quo uncertainty. You are, you are stuck in the middle. Now, there are really two distinct flavors of this flaw that you have to watch for on test day. The first one is the simple version we just described: taking no evidence for X, and leaping straight to therefore X is false.

Nora Ashford

Okay, and, and what is the second flavor?

Adrian Calloway

The second flavor is a bit more subtle. It is when an arguer takes one specific, terrible argument for X, completely dismantles that flawed argument, and then concludes that X itself must be false.

Nora Ashford

Oh, like, if a bad lawyer brings in a terrible, completely unreliable witness to prove the defendant was at the bank at three o clock, and you show that witness is lying, you have not proven the defendant was somewhere else!

Adrian Calloway

You have only proven that particular witness is a liar. The defendant might still have been at the bank! Knocking down a bad premise leaves the conclusion unproven, not disproven.

Nora Ashford

Unproven, not disproven. I, I literally wrote that on a sticky note on my monitor for two months.

Adrian Calloway

So how do we spot this in the wild under time pressure? You want to look for what I call the sharp about face in the stimulus. The premise section will be spending all its energy tearing down a single study, or calling a expert biased, or pointing out a flaw in a survey methodology. It is all about the weakness of the evidence.

Nora Ashford

Right, tear, tear down the study, attack the survey.

Adrian Calloway

And then, boom, you hit the conclusion, and the author makes a dramatic jump from this evidence is weak to therefore, this claim is categorically false or this event cannot happen.

Nora Ashford

That about face! It goes from, uh, we do not know, straight to, we know it is definitely not true!

Adrian Calloway

That pivot is your signal. And when you see that structural pivot, you immediately go to the answer choices looking for abstract descriptions of that mistake. LSAC loves phrasing it like, conflates a lack of proof for a claim with proof that the claim is false. Or, treats failure to establish a position as conclusive evidence against that position.

Nora Ashford

They have like, five standard, super legalistic ways of saying the exact same logical error, do not they?

Adrian Calloway

They really do. It is the same skeleton dressed in different abstract vocabulary every time.

Nora Ashford

Okay, so here is the practical test I used when I was stuck between two choices, especially on those tough Level Four and Level Five Logical Reasoning questions. I call it the test of replacement.

Adrian Calloway

Tell me about the test of replacement.

Nora Ashford

You scratch out the author's dramatic, sweeping conclusion, and you replace it in your head with, therefore, we remain uncertain, or therefore, we do not know yet. If that replacement conclusion actually follows logically from the premises, then you know the author committed the Absence of Evidence fallacy!

Adrian Calloway

Ah, that is brilliant, Nora, because if we do not know yet is the actual legitimate deduction, then jumping to definitely false exposes the unearned gap immediately.

Nora Ashford

Exactly! If the premises only justify saying we do not know, but the author said it is false, you have caught them red handed committing the flaw. You just grab the matching abstract answer choice and move on in forty five seconds.

Adrian Calloway

Forty five seconds. Clean, repeatable, no drama.

Nora Ashford

And no overthinking. Unproven stays unproven.