6 Identity and existence
In the preceding chapters we have set up a theory of first order logic allowing us to reason with descriptions of domains of things, given a vocabulary for representing relationships between them and expressions for quantifying over them. The logical system has been developed both syntactically as a calculus of natural deduction and semantically in terms of interpretations, with semantic tableaux and the sequent calculus to facilitate semantic reasoning.
All of that is fine and good, but to express much of what we would like to say, we need to extend the language by adding a notation for equations. This is familiar from Logic for Fun, where the very first thing we express is that x+2 = 4: equations are everywhere in practically useful applications of logic. In this chapter, we consider adding the obvious notation for identity, and go on to examine some related issues concerning existence, and especially how it is possible to reason about nonexistent things.
 Identity Reasoning with equations
 Definite Descriptions The logic of the word 'the'
 Presupposition Another view of definite descriptions
 Nonexistence An outline of free logic
 Examples Proofs with identity and proofs in free logic

Free logic proof exercises Sample problems with solutions