Sidney's broad defense of republican principles culminated in the articulation of a distinctly modern form of radical Whig republicanism. In his synthesis of Machiavellian republicanism and the modern natural rights theories of Hobbes, and especially of Spinoza, Sidney presents a vision of a new English republic grounded on radical Enlightenment philosophy. This new radical Whig English republic would be ruled by broadly based popular representative institutions that would immediately and directly reflect the sovereign power of the people. Sidney's republican idea of sovereignty is in a sense as absolutist as the moderate Whig idea of parliamentary sovereignty of king-in-Parliament; however, with the republican Sidney, the sovereign power is emphatically popular, even democratic. As such, Sidney offers a stinging critique of executive prerogative, and of the mixed and limited monarchical models of constitutionalism prevalent in England in the seventeenth century. In this respect, Sidney's vision of a democratic English republic broke new and uncharted ground in the Anglo-American constitutional and political tradition.

Republican Natural Rights

The core of Sidney's radical Whig republicanism is the idea of natural rights. These are the rights and liberties of individuals, which are “innate, inherent, and enjoy'd time out of mind.” The character of these rights “subsists as arising from the nature and being of man.” They derive from a source independent of their particular civil or historical context. The ground for Sidney's version of rights is the natural equality existing “in a multitude that is not entered into any society.