An assumption is simply an unstated premise in an argument – though it’s not mentioned, it’s necessary for the argument’s conclusion to be true.

Here’s an example:

On Tuesdays the sandwich shop closes at 5. It’s 5:30, so the sandwich shop is closed.

What’s needed in order for the example’s conclusion to be true? It must be Tuesday. This is the assumption. If the assumption is false, and today is actually Wednesday, the conclusion doesn’t follow from the premise, and nothing definitive can be reasoned.

We use assumptions all the time when we offer opinions and make suggestions or arguments. Rarely does this cause problems because often the assumptions we use are widely accepted beliefs or common knowledge that’s not needed to be made explicit. (The speaker in the example above would not have to state that today is Tuesday in order for the conclusion to be believable.)

Unlike everyday speech, GMAT critical reasoning questions, and good thinking for that matter, hinge on your ability to flesh out hidden assumptions in an argument. On questions that prompt you to strengthen or weaken the argument, find the flaw, find the assumption, or evaluate the argument, you’ll first need to find the argument’s assumptions in order to evaluate the answer choices.

Finding hidden assumptions requires two steps:

1. Identifying an argument’s premise(s) and conclusion(s)
2. Looking for a shift in scope or a gap in reasoning between the premise(s) and conclusion(s)

Once you have an understanding of an argument’s structure, you can examine how well the conclusion follows from the premises. This is where you’ll find the assumptions.

Look for the shift and you’ll find the assumption

All critical reasoning arguments contain at least one assumption. Even if premises and conclusions appear logically connected, an argument will still depend on unstated information.

Let’s start with an example that has an obvious shift between its premise and conclusion.

All opiate-based painkillers are addictive. Therefore, the drug Euphoripham, is addictive.

A shift in scope occurs between the premise (painkillers are addictive) and the conclusion (Euphoripham is addictive). The premise concerns all opiate-based painkillers and the conclusion talks about a single drug. What’s missing is a premise that connects all opiate-based painkillers  to Euphoripham: the assumption, Euphoripham is an opiate-based painkiller.

All opiate-based painkillers are addictive. Euphoripham is an opiate-based painkiller. Therefore, Euphoripham is addictive.

Usually, the gap or shift between premises and conclusions will be smaller. Assumptions become harder to spot when premises and conclusions are very similar and the premises appear to contain all the information needed to support the conclusion.

Here’s a more difficult example:

An invasive species of beetle that kills a variety trees with devastating speed has been found in the state park. Therefore, the park’s forest is threatened, and we must use all available resources to eliminate this pest as quickly as possible.

The premise of a tree-killing beetle’s presence in a state park seems to lead naturally to the conclusion that the forest faces imminent danger and that desperate measures are required. However, notice the shift between the premise and the conclusion. The premise concerns the threat the beetle poses to a variety of trees and the conclusion applies this threat to a forest. The assumption is that the forest is made up of the variety of trees the beetle kills.

This unstated premise is necessary for the conclusion to be true. If this assumption is false, and the beetle only kills elms, oaks and poplars and the park’s forest consists only of pine trees, there is little reason to believe that the forest is threatened.

Going from assumptions to answers

Identifying the hidden assumption is like finding an argument’s weak spot. Anything that counters the assumption weakens the argument, and anything that suggests or confirms the assumption strengthens the argument. If an assumption in an argument is weak to begin with, you’ll know that the argument is flawed, and more importantly, you’ll know exactly why it’s flawed.

This is exactly what you need to know in order to find the correct answer to the majority of critical reasoning prompts, so learn to find the shift in arguments and uncover the assumptions, and your critical reasoning abilities will improve dramatically.