  This theorem may have the longest pedigree of any theorem of model theory, since it generalises the Laws of Distribution for syllogisms, which go back at least to the early Renaissance. The theorem is easiest to state if we assume that our first-order languages have symbols ∧, ∨ and ¬, but not → or ⇔. Then an occurrence of a predicate symbol R in a formula φ is said to be positive (resp. negative) if it lies within the scope of an even (resp. odd) number of occurrences of ¬. 