Abstract

I am going to describe a variant formulation of classical extensional first-order logic and contrast it with the standard formulation. The formulation I will give is in one clear sense equivalent to the standard one, and it is a routine task to show that it is equivalent to it in this sense. So one might regard my formulation as a mere notational variation. But there are also ways in which the two formulations I will contrast are not equivalent, and I will argue that these ways reflect an important conceptual difference between them. The differences become more important as one generalizes or extends classical logic in various ways, but they are there even in the classical systems.