How to dissect LSAT formal vs. informal logic

Just like there is a big difference between what you can wear to McDonald’s and Gordon Ramsay’s new restaurant, there’s a big difference between the kinds of logic you’ll encounter on the LSAT.

Ok, so the metaphor was a bit of a stretch, but I’m doing my best here. As you might have guessed from that awkward metaphor, we’re gonna be talking about formal vs. informal logic today.
This is a huge part of the Logical Reasoning section, so making sure you understand these concepts is important.

Formal logic is easy enough. This is logic that is straightforward and relatively simple. Think Data from Star Trek or Spock…also from Star Trek.

For example: Everyone who lives on Earth lives in the Solar System. Everyone who lives in the Solar System lives in the Milky Way Galaxy. Therefore, everyone who lives on Earth lives in the Milky Way Galaxy.

I don’t know why I’m on a space kick today, but hey, space is cool.

While most formal logic on the LSAT is in the Logic Games section, you will see a bit of it in the Logical Reasoning section, too.

Usually these have something to do with morality or climate change. My article about frequent LSAT Logical Reasoning topics covered that.

Ok, let’s move on to what you’ll see way more often: informal logic questions.

Informal logic arguments tend to be more complex and contain unstated assumptions. These are tough to spot immediately because…well, because they don’t state them directly. And they can’t really be diagrammed.

For example:

Some people on Earth are aliens in disguise. John has a fascination with UFOs, secretively tinkers with machines in his garage and allegedly abducted a cow once. Therefore, John’s an alien in disguise.

John may very well be an alien in disguise, but this messy argument doesn’t prove it. You may even see filler sentences in there as well that don’t contribute to the argument at all but are just there to trip you up:

Some people on Earth are aliens in disguise. This is a fact that was unearthed by the CIA in 1947 at Roswell and carefully covered up. John, your reclusive neighbor, has a fascination with UFOs, secretively tinkers with machines in his garage and allegedly abducted a cow once. Therefore, John is an alien in disguise.

(The underlined sentence is an example of background / filler.)

This is a ridiculous example, but you get the point.

And that’s the basic gist of it. If you have any questions, you can always reach out to me.


Steve “The LSAT Czar” Schwartz

P.S. Click this link to check your "answers" to the free worksheet I shared the other day. (File --> Download As PDF) (Btw, at that link, I'll talk more about why I keep weirdly putting "answers" in quotes.)

P.P.S. Missed the worksheet? Didn't do it? I get it, we're all busy sometimes. Here's the link to give it another shot.

Recommended Resources:

1. LSAT CoursesThe best of my LSAT material with exclusive access to attend my Live Online LSAT Master Classes + Q&As, and on-demand video lessons you can watch anytime. Plus, LSAT study plans to keep you on track. Save hundreds of dollars with an LSAT course package.
2. Logical Reasoning Explanations
The explanations that should have come with the LSAT. These don't just fall back on "out of scope," but actually tell you why the wrong answers are wrong, why the right answers are right, and the easiest way to get the correct answer.

3. Logical Reasoning Cheat Sheet
Based on what I'd typically do in college: read what the professor emphasized and condense it all onto a single piece of paper. It gave me a quick reference, making things a lot less threatening and a lot more manageable.

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