"An existential graph is a logical graph governed by a. system of representation founded upon the idea that the sheet upon which it is written, as well as every portion of that sheet, represents one recognized universe, real or fictive, and that every graph drawn on that sheet, and not cut off from the main body of it by an enclosure, represents some fact existing in that universe, and represents it independently of the representation of another such fact by any other graph written upon another part of the sheet, these graphs, however, forming one composite graph." ('Logical Tracts. No. 2. On Existential Graphs, Euler's Diagrams, and Logical Algebra', CP 4.421, c. 1903)

"An existential graph is a logical graph constructed upon a perfectly consistent system of representation such that any unenclosed partial graph shall asserts something asserted by the entire graph." ('Logical Tracts. No. 1. On Existential Graphs', MS 491: 2, c. 1903)