Formal vs. Informal Logic in LSAT Logical Reasoning

Formal logic on the LSAT is simple and mathematical: the evidence fully proves the conclusion, no gaps. Informal logic is what you'll see most of the time. It's wordier and leans on assumptions nobody states, so the evidence never quite proves the conclusion.

As we saw in my interview with Dr. Deborah Bennett (author of Logic Made Easy), both formal and informal logic are necessary in everyday life. Here are some examples of both formal and informal logic for those of you who haven’t had a chance to get your hands on a copy of Logic Made Easy yet.

 

Formal logic
Formal arguments tend to be simple, straightforward, and extreme.

Example:

Everyone in Manhattan lives in NYC. Everyone in NYC lives in New York State. Therefore, everyone in Manhattan lives in New York State.

 

There are no assumptions here – it’s mathematical, and the evidence fully justifies the conclusion.

 

Represented in symbols, we can therefore say:
Manhattan -> NYC. NYC -> NYS. Therefore, Manhattan -> NYS.

 

Change the topic to something about climate change or morality, and you’ve got one of the few formal logic questions in Logical Reasoning. (See my article on 15 Common Logical Reasoning Topics for more on that). Formal logic used to live mostly in Logic Games. Since LSAC dropped that section in August 2024, the handful of formal questions in Logical Reasoning are all that's left of it.

 

On the LSAT, of course, it might not be that simple. The argument above could be phrased as follows

If you live in Manhattan, then according to accurate, yet decades-old, government records, you must live in NYC. However, if you’re in New York State, then you may or may not be in NYC. On the other hand, if you’re in NYC, then you must live in New York State.

I included the 2nd sentence as filler just to make the argument more difficult to understand. Although it’s more casual and wordy than the formal logic version, this doesn’t mean it’s easier.

The two versions above are identical. It’s not necessary to represent it in symbols, but it can sometimes help.

 

Informal logic
Informal arguments are much more common on the LSAT. They tend to be complex and contain unstated assumptions.

 

Example:

Some people in New York State aren’t famous. However, because I live in NYC, I ride around Manhattan in limos and hang out with celebrities. Therefore, I’m famous by association.

This can’t be diagrammed as neatly, the evidence doesn’t fully justify the conclusion (by a long shot), and a lot of other things also need to be true in order for the conclusion to logically follow.

Frequently asked questions

What's the difference between formal and informal logic on the LSAT?

Formal logic is mathematical and airtight: everyone in Manhattan lives in NYC, everyone in NYC lives in New York State, so everyone in Manhattan lives in New York State. Informal logic is wordier, more common, and rests on assumptions that go unstated. The formal kind runs on sufficient and necessary conditions, which is worth learning before anything else in LR.

Is most LSAT logic formal or informal?

Informal, by a wide margin. Formal logic is now a handful of Logical Reasoning questions. Before August 2024 most of it sat in Logic Games, which no longer exists.

What does an informal logic argument look like on the LSAT?

Something like: I live in NYC and hang out with celebrities, so I'm famous by association. It can't be diagrammed cleanly, and a lot has to be true that the argument never says for the conclusion to follow.

Can a formal logic argument be disguised on the LSAT?

Yes. The test writers dress a simple formal argument in casual, wordy filler so it reads harder than it is. Strip the filler and the underlying logic is unchanged.

Watch a related lesson

I've written free explanations for every question in LSAT PrepTest 159, 158, 157, 156, 155 and 154, with the right answer and why each wrong answer fails. All LSAT explanations are here.

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