Foundations of Logical Reasoning Arguments

TL;DR

Logical reasoning arguments on the LSAT consist of premises that support a conclusion. Your main job is to identify these parts and understand how they're supposed to connect. Flawed arguments often involve faulty connections between the premises and the conclusion.

1. The Mental Model

Think of an argument like building with LEGOs: premises are the individual bricks, and the conclusion is the structure you're trying to build. Your goal is to see if the bricks truly hold up the whole structure.

2. The Core Material

Every LSAT logical reasoning question is built around an argument or a set of statements that look like an argument. Your first step is always to identify its core components: the conclusion and the premises.

Identifying the Conclusion

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The conclusion is the main point the author is trying to convince you of. It's the central claim being argued for. Think of it as the "takeaway message."

You'll often find conclusion indicators like:
* therefore
* thus
* hence
* so
* consequently
* it follows that
* clearly
* as a result
* this shows that
* should / ought (for prescriptive conclusions)

Identifying the Premises

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The premises are the evidence, facts, or reasons the author provides to support their conclusion. They're the building blocks that lead to the main point.

You'll often find premise indicators like:
* because
* since
* for
* given that
* for example
* due to
* as shown by
* the reason is that

Unstated Assumptions

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Sometimes, an argument relies on an unstated assumption. This is an unwritten premise that must be true for the conclusion to logically follow from the stated premises. It's the "missing link" in the argument's chain. You won't see assumption indicators because, by definition, they're not stated!

graph TD
    A["Stated Premise 1 (Evidence)"] --> C["Unstated Assumption (Missing Link)"]
    B["Stated Premise 2 (More Evidence)"] --> C
    C --> D["Conclusion (Main Point)"]

Argument Structure & Types of Questions

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Understanding these parts is crucial because different question types ask you to interact with them in specific ways:

• Main Conclusion: Identify the conclusion.
• Weaken: Attack a premise or the link between premises and conclusion.
• Strengthen: Reinforce a premise or the link between premises and conclusion (often by confirming an unstated assumption).
• Flaw: Describe why the premises don't adequately support the conclusion (often due to a faulty assumption or reasoning error).

3. Worked Example

Consider this argument:

"The city's new bike lanes have caused a significant increase in local traffic congestion. Therefore, to reduce congestion, the city should remove the bike lanes."

• Conclusion: "to reduce congestion, the city should remove the bike lanes." (Indicator: "Therefore")
• Premise 1: "The city's new bike lanes have caused a significant increase in local traffic congestion."
• Unstated Assumption: Removing the bike lanes will actually reduce congestion, and there are no other significant causes of congestion that would remain. The argument assumes that the bike lanes are the sole or primary cause and their removal would fix the problem.

If you were asked to weaken this argument, you might say: "Congestion has also increased dramatically in nearby towns without bike lanes." This would attack the premise that bike lanes caused the congestion or, more precisely, attack the assumption that they're the only cause.

If you were asked to strengthen it, you might say: "A study showed that bike lanes in other cities consistently led to permanent traffic increases." This bolsters the link between bike lanes and congestion.

If you were asked for a flaw, you might point out that the argument assumes removing the identified cause will automatically solve the problem, ignoring other potential factors.

4. Key Takeaways

• Every argument has a conclusion (what the author wants you to believe) and premises (why you should believe it).
• Indicator words are incredibly helpful for quickly finding premises and conclusions.
• An unstated assumption is a hidden, necessary premise for the conclusion to follow logically.
• Understanding the structure helps you know where to focus for different question types.
• Weaken questions attack the support; strengthen questions reinforce it.

• Flaw questions pinpoint why the support is insufficient.

• Common mistakes to avoid:

  • Confusing a premise for the conclusion, or vice-versa.
  • Missing unstated assumptions that are critical to the argument's validity.
  • Focusing on irrelevant details instead of the core logical connection.
  • Assuming every statement in a passage is a premise or conclusion; sometimes there's just background info.

5. Now Try It

Take any newspaper editorial or persuasive advertisement. Read it once to get the gist. Then, go back and identify the single main conclusion the author wants you to accept, and list all the premises they use to support it. Write down any unstated assumptions that must be true for the argument to hold. Success looks like accurately identifying the core argument structure.