This paper examines Charles Peirce's graphical notation for first-order logic with identity. The notation forms a part of his system of "existential graphs," which Peirce considered to be his best work in logic. In this paper a Tarskian semantics is provided for the graphical system.

Several accounts of logical truth are compared and shown to define distinct concepts. Nevertheless, conditions are given under which they happen to declare exactly the same sentences logically true. These conditions involve the variety of objects in the domain, the richness of the language, and the logical resources available. It is argued that the class of sentences declared logically true by each of the accounts depends on particularities of the actual world.