``Mathematicians care no more for logic than logicians for mathematics.'' [Augustus de Morgan, 1868]
Proofs are traditionally syntactic, inductively generated objects. This paper presents an abstract mathematical formulation of propositional calculus (propositional logic) in which proofs are combinatorial (graph-theoretic), rather than syntactic. It defines a combinatorial proof of a proposition P as a graph homomorphism h : C -> G(P), where G(P) is a graph associated with P, and C is a coloured graph. The main theorem is soundness and completeness: P is true iff there exists a combinatorial proof h : C -> G(P).