Formal vs. Informal Logic in LSAT Logical Reasoning

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). Most formal logic on the LSAT happens in Logic Games.

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.