“Satire is a sort of glass, wherein beholders do generally discover everybody's face but their own;”
Swift jokes that readers readily recognize other people in satire while overlooking themselves, making self-exemption part of the very behavior satire exposes.
Source details
The Battle of the Books, 'The Preface of the Author,' represented by Project Gutenberg #623. Swift's 'satire is a sort of glass' image opens the Preface and explains why readers recognize other people's faults more readily than their own. The retained semicolon marks a frozen excerpt from a longer sentence; this is satirical/literary metaphor rather than a psychological measurement rule.