Logic

This is the part of the formalization of mathematics that deals only with the propositions.

In some systems, e.g. including Metamath, modus ponens alone tends to be enough, everything else can be defined based on it.

Builds on top of propositional logic, adding notably existential quantification.

Models existence in the context of the formalization of mathematics.

Existence and uniqueness results are fundamental in mathematics because we often define objects by their properties, and then start calling them "the object", which is fantastically convenient.

But calling something "the object" only makes sense if there exists exactly one, and only one, object that satisfies the properties.

One particular context where these come up very explicitly is in solutions to differential equations, e.g. existence and uniqueness of solutions of partial differential equations.