How does replacing nonlogical terms with variables reveal an argument’s logical form?

Replacing nonlogical terms with variables reveals an argument’s logical form by stripping away specific subject matter to expose the underlying structure that guarantees validity. In this process, all nonlogical terms—such as common nouns or predicates—are replaced by variables of the appropriate type, while logical constants (like "every," "some," "if–then," or quantifiers) are kept intact.12

This transformation results in a formula or inference form that represents the argument's skeleton. Because the validity of a deductive argument depends on its structure rather than its content, this method allows logicians to determine if an argument is valid by examining whether every uniform substitution of the variables that makes the premises true also makes the conclusion true. If no such substitution can result in true premises and a false conclusion, the form is considered valid. By abstracting away from particular matters of fact, this technique highlights the formal relations that ensure the necessity of the conclusion, independent of the specific entities or properties discussed.21