The Knewton Blog

This post was written by Kristen Kennedy.

On the LSAT Logical Reasoning section, an assumption is something in an argument that is not stated outright, but must be true for the argument to be valid. In other words, it is an idea that the author takes for granted when forming his/her argument.

So if assumptions aren’t stated outright in arguments on the LSAT, how are you supposed to identify them in Assumption questions? Here are the steps you should take first.

Take this simple argument as an example:

James cannot go to the store because his car is in the repair shop.

In this argument, the author says two things explicitly:

1. James cannot go to the store.
2. James’ car is in the repair shop.

As always, your first step should be to identify the conclusion of the argument. What point is the author trying to convince us of? The fact that James cannot go to the store.

Next, identify what is stated in the author’s reasoning. Most people who make arguments on the LSAT (and in real life) offer some sort of support for their claims. We call this stated support “evidence.”

To find the evidence, ask yourself: Why should we believe what the author’s telling us? According to this author, why can’t James go to the store? Because his car is in the shop.

So we would outline the stated argument like so:

Evidence: James’ car is in the repair shop.
Conclusion: James cannot go to the store.

Now let’s start thinking about the author’s assumptions. Assumptions are ideas that the author has not stated outright, but nevertheless must be true in order for the author’s conclusion to hold water.

In the most general terms, the assumption in the author’s argument is this:

James’ car being in the shop automatically means that James cannot go to the store.

Notice: The author never said that idea outright, but it’s the idea that’s central to the author’s argument. When she makes the claim that James won’t be able to go to the store, she’s assuming that his car is the only thing that can get him there.

A number of specific assumptions in the argument could fall under this general idea. Things like:

James cannot successfully take public transit to the store.
James cannot successfully ride a bike to the store.

Or, the assumption could be thought of a little more generally:

James cannot successfully travel to the store via some means other than his car.

So you see, even a simple argument may contain multiple ideas that the author takes for granted: ideas that she does not state outright, but that must be true in order for her conclusion to be true. These are assumptions.

Now, just for fun, let’s imagine an argument where the author spells everything out. Something like this:

James cannot go to the store, because his car is in the shop and whenever James’ car is in the shop it is impossible for James to go to the store.

This argument is logically stronger than the first because the author has taken the time to explicitly lay down each step of her argument. She now has two pieces of evidence. We would outline her argument as:

Evidence: James’ car is in the repair shop.
Evidence: When James’ car is in the shop, it is impossible for James to go to the store.
Conclusion: James cannot go to the store.

Notice how strong this argument is? When trying to build a strong argument (which you’ll need to do in law school and your legal career!), it’s always best to spell out each logical step of the argument. Think of the explicitly stated pieces of evidence as stepping stones that lead you all the way to the conclusion.

Conversely, when you are trying to attack an argument (on the LSAT and as a lawyer), look for ideas that have not been explicitly stated but that are necessary to your opponents’ arguments. Point out these assumptions and you’ll prove your logical supremacy in no time.

Posted in Logical Reasoning Guide, LSAT, LSAT Tips | One comment