The Rational Constitutionalism of Emmanuel Sieyès - This paper deals with the growing number of recent publications devoted to Emmanuel Sieyès political philosophy. By engaging with the new interpretations proposed in these works, this paper intends to shed a light on the deep unity one can detect between Sieyès philosophical approach and his constitutional theory. It is argued, in particular, that Sieyès’ constitutionalism is rooted in his philosophical rationalism. This means, first, that Sieyès constitutional theory is not fully shaped by historical events. Second, the aim of constitutionalism is to build a lawmaking process which is able to take into account the people's needs and to secure human rights. In this respect, Sieyès conception of constitutionalism implies a peculiar understanding of the principle of separation of powers, that is, one where different organs are cooperative, and not conflicting, parts in the lawmaking process.