This paper reconstructs Venn’s algebraic logic and identifies some of the philosophical notions concerning the nature of symbolic knowledge underlying his work. We show that Venn, in facing philosophical problems associated with his algebraic logic, needs to articulate the symbolic knowledge notions of ecthetic function and of surrogative function. The paper explains those notions based on the systematization of the functions of symbolic knowledge that we find in the recent philosophical literature. This paper also situates Venn’s work within the 19th Century efforts towards a symbolization of logic, and, through an analysis of a historical case, clarifies the notions of ecthetic function and of surrogative function.