Argumentation is a prominent approach for reasoning with (inconsistent) propositional information. It is based on the justification of formulas by arguments, which are minimal and consistent logical proofs of the formulas. The aim of this paper is to evaluate to what extent two such arguments are similar. For that purpose, we introduce a notion of similarity measure and a set of principles that such a measure should satisfy. We propose some intuitive extensions of measures from the literature, and show that they fail to satisfy some of the principles. Then, we come up with a more discriminating measure which satisfies them all.