Syllogistic Fallacies

This lesson provides a detailed examination of the logical fallacies that appear most frequently in UCAT Decision Making syllogism questions. Understanding why these errors are tempting — and why they are logically invalid — is the key to avoiding them under time pressure. Each fallacy is explained with its formal structure, a concrete example, and a Venn diagram analysis showing exactly where the reasoning breaks down.

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Why Fallacies Matter in the UCAT

The UCAT tests whether you can distinguish valid from invalid reasoning. The test designers deliberately construct answer options (and proposed conclusions in Yes/No items) that commit well-known logical fallacies. These fallacies are chosen because they are psychologically persuasive — they feel right even though they are logically wrong.

Your defence is to learn the fallacy patterns so thoroughly that you recognise them instantly.

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Fallacy 1: Undistributed Middle

Formal Structure

Premise 1	All A are C
Premise 2	All B are C
Fallacious conclusion	Therefore, some A are B

Why It Is Invalid