Core Assumptions and Unstated Premises

TL;DR

Assumptions are unstated facts that must be true for an argument's conclusion to logically follow from its premises. Identifying assumptions is key to weakening or strengthening an argument, as well as finding flaws. When an argument makes a leap, an assumption bridges that gap.

1. The Mental Model

Think of an argument as a bridge connecting a starting point (premises) to an ending point (conclusion). An assumption is like a hidden, necessary plank that allows you to cross the gap smoothly. Without it, the bridge falls apart.

2. The Core Material

On the LSAT, arguments often leave out a crucial piece of information – the assumption – that ties the evidence to the conclusion. Your job is to find what must be true, but isn't stated, for the argument to make sense.

What is an Assumption?

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An assumption is an unstated premise. It's something the author takes for granted. If an assumption isn't true, the argument fails. This is why it's so critical for Weaken, Strengthen, and Flaw questions.

How to Find Assumptions: The Negation Test

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The most reliable way to confirm if something is an assumption is the Negation Test:
1. Take a potential assumption.
2. Negate it (make it false).
3. If negating the statement destroys the argument's conclusion, then it's a true assumption. If the argument can still stand even if the statement is false, it's not a necessary assumption.

For example, if an argument concludes "Therefore, we should buy a new car" based on the premise "Our old car often breaks down," a necessary assumption might be "Buying a new car will solve the problem of the car breaking down." If you negate this ("Buying a new car will not solve the problem"), the conclusion falls apart.

Common Assumption Categories

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Assumptions often fall into certain categories:

• Causal Assumptions: Assuming that one event caused another, or that there are no other causes.
• Sampling/Generalization Assumptions: Assuming that a sample group is representative of a larger population.
• Analogy Assumptions: Assuming that two things being compared are similar in all relevant respects.
• Conditional/Necessary Condition Assumptions: Assuming that a certain condition is truly necessary for a result.
• Exclusivity/Completeness Assumptions: Assuming that all possibilities have been considered, or that no other factors are relevant.
• Values/Goals Assumptions: Assuming that a particular value or goal is shared or universally important.

graph TD
    P["Premise: Our old car often breaks down."] --> A["Assumption: Buying a new car solves the breakdown problem."];
    A --> C["Conclusion: We should buy a new car."];
    P -- "Logically connects to" --> C;
    A -- "Is the 'missing link' that enables" --> C;
    "Negate A"["Negation Test: Assume 'Buying a new car *won't* solve the problem'."] --> "Argument Fails"["Conclusion no longer follows."];

Assumptions in Different Question Types:

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• Weaken: You'll weaken an argument by attacking or disproving a key assumption.
• Strengthen: You'll strengthen an argument by providing additional support for a key assumption.
• Necessary Assumption: You're looking for the one statement that must be true for the conclusion to logically follow. (Use the Negation Test!)
• Flaw: Often, the flaw in an argument is that it relies on a questionable or unwarranted assumption.
• Main Conclusion: While not directly about assumptions, understanding the argument's structure helps you see what's being assumed.

3. Worked Example

Argument: "Professor Albright's history class uses only primary source documents. Therefore, students in her class will have a deeper understanding of historical events than students in Professor Bell's class, which uses only textbooks."

Task: Identify a necessary assumption.

Breakdown:
* Premise 1: Albright's class uses primary sources.
* Premise 2: Bell's class uses textbooks.
* Conclusion: Albright's students will have a deeper understanding than Bell's students.

There's a gap between "uses primary sources" and "deeper understanding," and between "uses textbooks" and "less deep understanding." The argument assumes something about the relative value of these materials.

Potential Assumption: "Using primary source documents always leads to a deeper understanding of historical events than using textbooks."

Negation Test: What if we negate this? "Using primary source documents does not always lead to a deeper understanding of historical events than using textbooks." If this is true, then the conclusion that Albright's students will have a deeper understanding falls apart. The argument is destroyed.

Therefore, the statement "Using primary source documents always leads to a deeper understanding of historical events than using textbooks" (or a very similar idea) is a necessary assumption.

4. Key Takeaways

• Assumptions are unstated facts that must be true for the argument to work.
• The Negation Test is your best friend for verifying necessary assumptions.
• Assumptions bridge the logical gap between premises and conclusion.
• Spotting assumptions is crucial for almost all LSAT logical reasoning questions.
• Assumptions often involve connections like causation, representativeness, or shared values.

Common Mistakes to Avoid:
* Confusing assumption with inference: An inference is something that can be concluded from the premises; an assumption is something that must be true for the conclusion to follow.
* Choosing something that strengthens but isn't necessary: The Negation Test helps here. If the argument can still stand without it, it's not a necessary assumption.
* Selecting an option that is too strong or too weak: Pay attention to qualifying words like "always," "some," "only," or "most."
* Overlooking alternative causes/explanations: Many arguments assume there's only one cause or that no other factors are relevant.

5. Now Try It

For the following argument, identify one necessary assumption and confirm it using the Negation Test.

Argument: "Our company will increase profits next year because we are investing heavily in a new advertising campaign designed to reach a broader customer base."

What to do:
1. Identify the premise(s) and conclusion.
2. Look for the logical leap or gap between them.
3. Formulate a statement that bridges that gap.
4. Apply the Negation Test to your statement.

What success looks like: You should be able to articulate a clear assumption, and when you negate it, the original conclusion (increased profits) should no longer logically follow from the given premise (new ad campaign).