The Pillar of Assumptions: Necessity

May 28, 2013

Last week we took a stroll back in time and looked at the disco age and remembered Donna Summer’s song “Enough is Enough”. In the context of the LSAT it is important, shall we even say necessary, to be able to distinguish between elements of an argument that are sufficient from those that are necessary. In formal logic, statements are often couched in terms that say something is required, or an element is essential. To correctly form the formal logic the term that is required is placed on the right side of the arrow, and becomes the necessary term. Often, the test-makers will challenge those taking the LSAT by posing answer choices with terms reversed.  It is important to remember that something that is necessary is not always sufficient. For example, gas is essential in order to drive a car; can we say that if we have gas we can drive the car?  Of course not – gas in the car doesn’t mean that the engine is running, or even if there is an engine; maybe the spark plugs are gone or the battery is dead.

In addition to formal logic, terms of necessity and sufficiency arise in other areas of the LSAT. The terms play an important role in assumption questions when in the question stem. Often we see questions asking for what could be an assumption; in other question stems, we are asked for an assumption on which the argument depends or to identify a necessary assumption. Taking the time to identify the difference in the question stem can often lead to different strategies to help identify the assumption, and ultimately to higher points.

Imagine a three-legged stool: the legs are essential to maintaining the stool’s balance; remove one leg and the stool falls over. So it is with necessary assumptions; if the assumption is taken away, the argument falls apart. Understanding this can help determine whether an answer choice in an assumption question is correct.

It is always good test protocol to try and predict an answer choice. If unable to predict, or facing uncertainty, an answer choice can be tested using what we at Kaplan call the “denial” test.  Facing a question asking about a necessary assumption, take a minute to look at the answer choices and then see what happens if you deny the truth of that answer choice. If there is no effect, then the choice cannot be correct; however, any answer choice, which, when denied, results in the argument falling apart must be correct!

As you read through the question stem in assumption questions, take a minute to try and identify if it’s asking for a necessary assumption. Using the denial test takes time and practice;while it is a useful tool to help to determine the answer choice of a question remember that by itself it is not sufficient for a high score. Practice and repetition are always essential tools for high LSAT scores.