The aim of this paper is to make a comparison between two philosophical theories on convention, totally independent from each other, and produced by two thinkers of this century yet totally unknown to each other, in order to assess to what extent both theories meet the same facts or are verified by the same features. The result of this inquiry is that each theory is autonomous, but that both theories are quite compatible with each other, that they even fairly coincide (...) with each other, and that there where they emphasize different aspects of a convention they are complementary to each other. In a general introductory paragraph we lay stress upon the distinction between something conventional and something arbitrary and assign a place to conventionalism between rationalism and relativism. In the second part of the paper we examine D. K. Lewis's conception and analysis of the notion of a convention, which he defines in terms of the theory of games of pure coordination, in terms of coordination equilibrium, of orders of mutual expectations, of preferences and of regularities. In the third part of the paper we show that Dupréel's sociological concept of 'being of one mind' is the analogue of Lewis's definition. We also compare Lewis's remarks on the phenomenon of the imitation with Dupréel's theory of the consolidation and endorse the latter's view that a convention has a creative function so that it cannot be reduced to being a pure sign of something else. In the fourth part of our paper we analyze the relation between value and convention and claim that in Dupréel's philosophy the main values may not be identified with conventions. Lastly we justify our conclusion as already stated at the beginning of this summary. (shrink)