Necessary Assumptions

Free LSAT Course > Logical Reasoning > Assumptions > Necessary Assumptions

As we discussed in the previous lesson, there are two types of Assumptions seen on LSAT questions: Sufficient Assumptions and Necessary Assumptions. These questions will typically ask to identify and fix the arguments within. So, these can also be referred to as Strengthening/Weakening Questions.

CHALLENGE: Identify the Necessary Assumptions

Necessary Assumption questions have stems phrased in the following ways:

The argument requires/depends on/relies on the assumption that…
Which of the following is an assumption on which the argument relies/depends?
Which one of the following is an assumption the argument requires?
The argument requires/depends on/relies on assuming which one of the following?

Notice that Necessary Assumption questions always contain a word that means necessary or required, while Sufficient Assumption questions ask for a guarantee that the conclusion will be reached.

Intro (0:01) | Ex.1 (5:53) | Ex.2 (18:16) | Trap Choices (24:40) | Summary (26:42)

Sufficient vs. Necessary Review

Next LSAT: January 16-17

{"first_video":"a0JbDI-FPgQ","testData":[{"videoId":"a0JbDI-FPgQ","videoLoopStart":"1.5","videoLoopEnd":"2.5","VideoEnd":"560","videoCustomization":"","correct":"D","explanation":"0:00 - This is a Necessary Assumption<\/i> question. The answer is going to be a claim that has to be true in order for the argument to work. | \n0:48 - Read the stimulus. Focus on understanding the main conclusion and evidence of the argument. And try to identify any gap between the conclusion and evidence. | \n1:35 - Conclusion:<\/b> If the government were to lower income taxes, the economy would improve. | \n2:05 - Evidence:<\/b> High prices lead to people not wanting to spend, which leads to economic weakness. And reluctance to spend is made worse by lower income. |\n2:50 - Gap in the Argument:<\/b> The conclusion makes a statement about income tax, but income tax is never discussed in the evidence. | \n3:30 - Assumption Prediction:<\/b> If the government were to lower income taxes, consumers would be more willing to spend. | \n4:40 - (A) does not match the prediction, so eliminate it. | \n5:05 - (B) does not match the prediction, so eliminate it. |\n5:30 - (C) does not match the prediction, so eliminate it. |\n5:55 - Negate (C) to check that it is incorrect. For Necessary Assumption<\/i> questions, if negating an answer choice weakens the argument, it is the correct answer. |\n7:40 - (D) matches the prediction. Test it by negation. Negating (D) makes the argument fall apart, so it is a necessary assumption. | \n8:23 - (E) is irrelevant to the argument. | \n8:35 - Recap.\n\n\n","explanationBreaks":"0:00 - This is a Necessary Assumption<\/i> question. The answer is going to be a claim that has to be true in order for the argument to work. \n0:48 - Read the stimulus. Focus on understanding the main conclusion and evidence of the argument. And try to identify any gap between the conclusion and evidence. \n1:35 - Conclusion:<\/b> If the government were to lower income taxes, the economy would improve. \n2:05 - Evidence:<\/b> High prices lead to people not wanting to spend, which leads to economic weakness. And reluctance to spend is made worse by lower income. \n2:50 - Gap in the Argument:<\/b> The conclusion makes a statement about income tax, but income tax is never discussed in the evidence. \n3:30 - Assumption Prediction:<\/b> If the government were to lower income taxes, consumers would be more willing to spend. \n4:40 - (A) does not match the prediction, so eliminate it. \n5:05 - (B) does not match the prediction, so eliminate it. \n5:30 - (C) does not match the prediction, so eliminate it. \n5:55 - Negate (C) to check that it is incorrect. For Necessary Assumption<\/i> questions, if negating an answer choice weakens the argument, it is the correct answer. \n7:40 - (D) matches the prediction. Test it by negation. Negating (D) makes the argument fall apart, so it is a necessary assumption. \n8:23 - (E) is irrelevant to the argument. \n8:35 - Recap.\n\n\n","clockPosition":"TR","imageThumb":"https:\/\/www.lsat-center.com\/wp-content\/uploads\/yt_thumbs\/45_a0JbDI-FPgQ.jpg","categoryId":"15","id":"45"},{"videoId":"KPxEE-vOMGs","videoLoopStart":"1.0","videoLoopEnd":"265.0","VideoEnd":"265","videoCustomization":"","correct":"E","explanation":"0:00 - Read the stimulus and question. Make a diagram of the situation. | \n1:30 - Assumption prediction:<\/b> The armadillos didn`t go anywhere between the spring and the fall. | \n1:55 - (A) is incorrect because the proportion of tagged armadillos is irrelevant. | \n2:20 - (B) is incorrect because it would actually undermine the researcher`s argument. |\n2:45 - (C) is incorrect because the researcher is not assuming that he had to recapture all the armadillos. |\n3:19 - (D) is incorrect for the same reason as (C). |\n3:52 - (E) matches the prediction and is the correct answer.\n\n","explanationBreaks":"0:00 - Read the stimulus and question. Make a diagram of the situation. \n1:30 - Assumption prediction:<\/b> The armadillos didn`t go anywhere between the spring and the fall. \n1:55 - (A) is incorrect because the proportion of tagged armadillos is irrelevant. \n2:20 - (B) is incorrect because it would actually undermine the researcher`s argument. \n2:45 - (C) is incorrect because the researcher is not assuming that he had to recapture all the armadillos. \n3:19 - (D) is incorrect for the same reason as (C). \n3:52 - (E) matches the prediction and is the correct answer.\n\n","clockPosition":"TR","imageThumb":"https:\/\/www.lsat-center.com\/wp-content\/uploads\/yt_thumbs\/104_KPxEE-vOMGs.jpg","categoryId":"15","id":"104"}]}