Discussion
What is a valid sentence?
Answer: C
A valid sentence, or tautology, is true whatever the symbols are assigned, so it carries no information about the world. Truth in some model is satisfiability, entailment is relative to premises, and connective count is a syntactic property. Validity, satisfiability and entailment are three notions defined by quantifying over models in three different ways, and confusing them is the commonest error in a first encounter with the semantics. Framing the question as which quantity the change acts upon makes the answer follow without calculation.
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