It is a well-known fact that Kant´s doctrine of the original social contract is so complex that it has prompted many different, sometimes opposing interpretations of his political philosophy. A simple enumeration of some of the central tenets of Kant´s ideal social contract theory suffices to let us see the reasons of that complexity. On one hand, (a) Kant claims that leaving the state of nature is a categorical duty (albeit moral in a juridical, not ethical, sense), and consequently, (b) he relies neither on instrumental-reasoning arguments, nor on the individuals´ subjective motivation to prove the practical necessity to leave the state of nature. Kant also held that (c) the social contract is to be considered exclusively as a norm, a regulative principle to be used as a normative criterion to establish the ideal Staatsrecht and to critically evaluate the legitimacy of existing states, rather than as a descriptive explanation of the origin of the state aiming at legitimising its empirical exercise of the monopoly of coercion. With these three theses, (d) the idea that juridical-political obligations have their origin in individual and unilateral choices no longer seems to be extant in Kant´s Doctrine of Right. Kant even states that in the state of nature "each may impel the other by force to leave this state and enter into a rightful condition" (RL, 312, § 44). Kant explains that the difference between a social contract and any other kind of contracts consists in that (e) regarding the "pactum unionis civilis", unity is an "end in itself", "an unconditional and first duty in all external relationships in general among human beings, who cannot avoid reciprocally influencing one another" (TP, 289). In the Doctrine of Right, Kant explains that unlike any other kind of society the principle "you ought to enter this condition" "holds a priori" for the juridical or rightful society. This principle is necessarily valid a priori for "all human beings who could (even involuntary) come into relations of right with one another" (RL, 306). This deontological modality of Kant´s exeundum separates him from the typical strategy of the social tradition of contrasting the empirical inconveniencies of the state of nature with the security of the rightful condition, and of consequently showing the prudentially rational, as opposed to practical and moral, modal character of the exeundum principle. But on the other hand, (f) the Doctrine of Right develops an argument for the practical necessity of the state that gives an account of the natural condition in order to determine the pressing problem the state authority is meant to solve, which is the key move of the social contract tradition?s argumentative strategy. Moreover, (g) this argument serves to justify the rule of the general will. In § 47 of this work, Kant stated that (h) a person´s "dependence upon laws" "arises from her own lawgiving will" (RL, 315-16), and in § 44 (h) he explicitly adopted the thesis of the artificiality of the state: The state, though conceived as an a priori ideal, must be created. It is uncontroversial to state that the way Kant combined these tenets in a cohesive doctrine of the original contract makes this theory a very original one, but disagreement starts when it comes to establishing where exactly in this theory its originality chiefly resides. I propose that the proper Kantian contribution to the social contract tradition is his concept of an a priori united will, a concept that aims at solving some problems Kant found within the two modern political traditions which he both adopts and criticizes and that do permeate liberal democratic theory up to this day. I am referring, of course, to modern natural law theories and to social contract theories, which were objects of Kant´s political reflexion since a very early stage in his intellectual life. My general aim in this essay is to critically compare Kant´s social contract theory with modern social contract tradition. I have chosen this path because of a simple reason: Against his own rendering of the problems of this tradition, Kant maintained that a solid theory of the state could only be based upon a purely juridical and political starting point. This is the main reason that allows us to detach Kant´s social contract theory from many contemporary contract doctrines that mistakenly consider themselves to be Kantian. I sustain that Kant´s social contract theory enjoys some features of its own kind that render it a fruitful theory also vis-à-vis contemporary contractualisms and contractarianisms, and so my more specific purpose is to try to find what Kant´s political philosophy contributes to our own contemporary conception of the social contract and its function.