The focus of this paper is on the justificatory basis of democracy. The paper operates on two levels, theoretical and historical. On the theoretical level the author claims that many of the arguments put forward to justify democracy, such as that formulated by Dahl in ‘Democracy and its Critics’ based on his two principles of equality, although not without merit, suffer certain crucial weaknesses and do not, in fact, get at the real basis of the belief in the unique legitimacy of democracy. He goes on to argue that this legitimacy is grounded not simply in the positive egalitarian consequences expected from democracy, but rather is to be found in the moral autonomy of the human being. Further, he claims, this moral autonomy is itself rooted in what the author calls the Cartesian autonomy of reason. On the historical level he claims that, while Descartes was himself extremely conservative with regard to orthodox Christian belief and traditional structures of political authority, many self-styled followers of Descartes saw the autonomy of reason as implying a radical rejection of all ‘external’ authority, first in respect of religious belief, but also, then, with respect to the secular authority. The result was that within what Jonathan Israel refers to as the ‘Radical Enlightenment’, there developed as early as the mid-seventeenth century a tradition of liberal and democratic radicalism, based explicitly on the Cartesian autonomy of reason and what was referred to as the ‘freedom to philosophise’. The author illustrates this with a brief account of the Dutch radical thinker Franciscus Van den Enden. He argues that if we posit moral autonomy as the basis of democratic legitimacy, this privileges one particular conception of democracy, namely deliberative democracy, as its paradigmatic form. Throughout the whole argument he gives a central role to the autonomy of reason as, in particular, it began to sweep across Europe with the influence of Cartesianism. It is possible that there are older egalitarian roots to modern democratic ideology or that democratic authority is grounded on democracy's epistemic properties. The author looks at these claims towards the end of the paper and concludes that the autonomy of the moral agent as based itself on the autonomy of human reason is the most plausible basis of the unique legitimacy of democracy.