Logic / Unit 6 Logic 262 short lesson, about 20 seconds
Free vs. Bound Variables
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The idea
A variable is bound when a quantifier's scope covers it, free otherwise. Statements have no free variables.
In practice
In (x)Fx ⊃ Gx, the second x is free — the quantifier's scope ended at Fx. Compare (x)(Fx ⊃ Gx).
Where people slip
Parentheses set quantifier scope exactly as they set connective scope (129). Same discipline, higher stakes.
Your place on the trail
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