Entitative graph

An entitative graph is an element of the graphical syntax for logic that Charles Sanders Peirce developed under the name of qualitative logic beginning in the 1880's, taking the coverage of the formalism only as far as the propositional or sentential aspects of logic are concerned. See 3.468, 4.434, and 4.564 in Peirce's Collected Papers.

The syntax is:
 * The blank page;
 * Single letters, phrases;
 * Objects (subgraphs) enclosed by a simple closed curve called a cut. A cut can be empty.

The semantics are:
 * The blank page denotes False;
 * Letters, phrases, subgraphs, and entire graphs can be True or False;
 * To surround objects with a cut is equivalent to Boolean complementation. Hence an empty cut denotes Truth;
 * All objects within a given cut are tacitly joined by disjunction.

A "proof" manipulates a graph, using a short list of rules, until the graph is reduced to an empty cut or the blank page. A graph that can be so reduced is what is now called a tautology (or the complement thereof). Graphs that cannot be simplified beyond a certain point are analogues of the satisfiable formulas of first-order logic.

Peirce soon abandoned the entitative graphs for the existential graphs, whose sentential (alpha) part is dual to the entitative graphs. He developed the existential graphs until they became another formalism for what are now termed first-order logic and normal modal logic.

The primary algebra of G. Spencer-Brown is isomorphic to the entitative graphs.