Quantifiers: For All & There Exists

"Every student passed." "Some student passed." These two sentences look similar but mean wildly different things — and getting them mixed up has crashed programs and voided contracts. Logic gives each its own symbol: ∀ means "for all" (every single one), and ∃ means "there exists" (at least one). Master these two and you can state any claim about a collection with zero ambiguity.