What is the Barbara Liskov quotation that defines LSP?

Barbara Liskov's formal definition from her 1988 paper states: "If for each object o1 of type S there is an object o2 of type T such that for all programs P defined in terms of T, the behavior of P is unchanged when o1 is substituted for o2, then S is a subtype of T." In simpler terms: a program that works correctly with a Bird object must work correctly when any subtype of Bird is substituted, without knowing the concrete type. This is a stricter definition than mere structural substitutability (the "is-a" relationship) — it demands behavioral substitutability. The principle emphasizes that subtyping is about behavior contracts, not just method signatures, and is what distinguishes LSP from mere type inheritance.