Contractualism

Political society is not a natural order or one willed by God, but a human institution born of the agreement of individuals: its sole source of legitimacy is the consent of the governed, not force, tradition or the divine right of kings. Locke makes it a revocable pact that the people may renounce if power betrays its end; Rousseau seeks the form in which, uniting with all, each « still obeys only himself » and remains free. Against this founding fiction Burke will set the organic community, inherited rather than contracted: but it is contractualism that gives the age of revolutions its grammar of legitimacy.

Rousseau, The Social Contract · Locke, Two Treatises of Government, II

No one has authority over another by nature: all being free and equal in the state of nature, no political power can be legitimate without the consent of those who submit to it. From this premise Locke draws a contract of an altogether different kind from Hobbes's: one does not surrender one's rights to an absolute sovereign, one entrusts government with a mere trust, bound to an end, the preservation of lives, liberties and estates. Power thus stays conditional and revocable, where the Hobbesian pact was an alienation beyond recall. This is the foundation of political liberalism.

Two Treatises of Government, II, ch. 8 ("Of the Beginning of Political Societies")

Since no one has natural authority over his fellow and force creates no right, the legitimate order can rest only on a convention. Rousseau seeks the form of association in which each, giving himself to all, gives himself to no one, and « still obeys only himself »: through each person's alienation to the general will, the subject remains free in obeying a law of which he is co-author. Against Hobbes, the pact institutes not a master but a sovereign body politic.

Rousseau, The Social Contract, books I-II

Justice is nothing absolute or natural in itself: it is a compact of utility made so as not to harm one another, varying with place and circumstance. This conventionalist conception, where the just reduces to a contract grounded in common advantage, is an early form of contractualism.

Principal Doctrines, XXXI-XXXVIII (justice as a compact of utility)

The civil condition rests on an original contract, not as a historical fact but as an idea of reason: the criterion of a just law is that it could have received the assent of the whole people. The legitimacy of power is grounded on this rational agreement, an avowed contractualism.

On the Common Saying: That May Be Correct in Theory, but It Is of No Use in Practice · Metaphysics of Morals, Doctrine of Right

Right and laws descend neither from the gods nor from a natural order: weary of the violence of private vengeance, men of their own accord consented to neighbourly pacts so as no longer to harm one another, and from that agreement justice was born. Lucretius thus gives, before the term existed, a contractual genesis of civil order, in which the just is a convention of common advantage and not a transcendent command, extending the compact of utility of Epicurus. This is a naturalist contractualism.

On the Nature of Things, V, 1011-1027 (neighbourly pacts) and 1136-1160 (the founding of law)