# Why second-order logic?

Wikipedia says: First-order logic quantifies only variables that range over individuals (elements of the domain of discourse); second-order logic, in addition, also quantifies over relations. For example, the second-order sentence ${\displaystyle \forall P\,\forall x(x\in P\lor x\notin P)} \forall P\,\forall x(x\in P\lor x\notin P)$.

Who has first published this doctrin? And why has it been accepted?