Common logical fallacies and reasoning errors - Errors of co...

Learning Outcomes

By the end of this article, you will be able to define and identify errors of composition and errors of division in LSAT reasoning. You will be able to distinguish these fallacies from related reasoning errors and explain their impact on the validity of arguments. You will understand how to approach LSAT questions dealing with these flaws and apply techniques to avoid common traps.

LSAT Syllabus

For LSAT, you are required to understand logical fallacies related to argument structure and reasoning. This article focuses your revision on:

• the definition and identification of errors of composition and division
• understanding how these fallacies undermine LSAT argument validity
• techniques for analysing and challenging arguments that contain these flaws
• distinguishing errors of composition/division from other related fallacy types

An accurate understanding of these flaws enables you to avoid attractive wrong answer traps and to select correct responses to flaw and assumption questions.

Test Your Knowledge

Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, remember to look more closely at that area during your revision.

1. Which of the following best describes an error of composition?
   1. Ascribing a property from members to the group as a whole without justification.
   2. Assuming part of a cause is the entire cause.
   3. Confusing correlation with causation.
   4. Relying on irrelevant evidence.
2. Which argument demonstrates an error of division?
   1. "This team is undefeated, so every player must be outstanding."
   2. "The pie was delicious, so every ingredient was tasty."
   3. "If one section of the law is unconstitutional, the whole law is unconstitutional."
   4. Both a) and b).
3. In LSAT flaw questions, errors of composition and division most frequently arise in arguments about:
   1. Syllogistic reasoning.
   2. Sample representativeness.
   3. Generalizing parts to wholes and vice versa.
   4. Predicting future events.

4. True or false? Proving that a group has a property is always sufficient to prove every member has it.

Introduction

Errors of composition and division are common reasoning flaws in LSAT arguments. These fallacies involve mistaken transfers of properties between parts and wholes. For the LSAT, you must spot these errors to evaluate and challenge flawed reasoning.

Key Term: error of composition
A reasoning flaw where an argument assumes that what is true of the individual parts must also be true of the whole group or entity.

Key Term: error of division
A flaw in reasoning where an argument assumes that what is true of the whole must also be true of each part or component.

Recognizing Composition and Division Errors

Errors of Composition

An error of composition occurs when an argument attributes a property of a group’s members to the group itself, or to a group as a collective. This is illegitimate, unless there is a justified link between the parts and the whole.

LSAT Clues: Often, the flaw is present when arguments move from “every member” or “each component” to “the group as a whole.”

Key Term: whole-to-part fallacy
A flaw where an argument assumes that, since a whole possesses a quality, every part must also possess that quality.

Key Term: part-to-whole fallacy
An argument error that assumes that if each part (member, component) has a property, so must the whole.

Errors of Division

Conversely, an error of division arises when an argument concludes that since the whole has a property, every part shares that property. This is not automatically true. The property may not be distributive.

LSAT Clues: Division errors frequently arise when a collective or group is proven to have a feature and the argument presumes this applies to every member.

Worked Example 1.1

Argument: "Every member of the orchestra is talented, so the orchestra must be a brilliant ensemble."

Answer:
This is not an error. If all members are talented, the group’s potential is high. However, if the argument said, "The orchestra is renowned, so every musician must be outstanding," this would be an error of division (assuming group quality is always reflected by each part).

Worked Example 1.2

Argument: "The university received a prestigious award for research, so every department is exceptional at research."

Answer:
This is an error of division. Just because the university as a whole has a distinction doesn't mean every department, individually, does.

Worked Example 1.3

Argument: "Each brick in the wall is light, so the whole wall must be light."

Answer:
This is an error of composition. Even if the parts are light, their sum (the wall) could be heavy.

Exam Warning

On flaw questions, be cautious of arguments that move from “every” or “all” to a group conclusion, or from a group conclusion to inferences about every member. Not all group properties distribute to parts and vice versa.

Distinguishing Composition/Division from Related Flaws

Errors of composition and division are specific. Do not confuse them with flawed analogies, sampling errors, or causation errors, even if some answer choices sound tempting. In composition/division, the central issue is the misapplication of properties between parts and whole.

Revision Tip

To spot these flaws quickly, underline shifts between “all,” “each,” “these,” or “the group.” Look for unproven leaps from part to whole or from group to member.

Summary Table: Composition vs. Division

Key Point Checklist

This article has covered the following key knowledge points:

• Errors of composition and division arise when properties are incorrectly assumed to transfer between wholes and parts or vice versa.
• On the LSAT, watch for arguments that assume the whole has an attribute just because a part has it, or the reverse.
• Not every property is distributive; some group features do not belong to the parts, and some features of parts do not add up to the same property for the whole.
• These flaws are distinct from analogy, sampling, or causation errors. Look for unwarranted shifts between parts and group.
• Recognizing these flaws enables you to reject flawed arguments in flaw, parallel, and assumption LSAT questions.

Key Terms and Concepts

• error of composition
• error of division
• whole-to-part fallacy
• part-to-whole fallacy