It is clear that, if such a theory is to be upheld, it can stand only by the view, which Hobbes shares with Grotius, that a man can alienate not merely his own liberty, but also that of his descendants, and that, consequently, a people as a whole can do the same. This is the point at which both Locke and Rousseau attack it. Locke, whose aim is largely to justify the Revolution of 1688, makes government depend, not merely at its institution, but always, on the consent of the governed, and regards all rulers as liable to be displaced if they govern tyrannically. He omits, however, to provide any machinery short of revolution for the expression of popular opinion, and, on the whole, seems to regard the popular consent as something essentially tacit and assumed. He regards the State as existing mainly to protect life and property, and is, in all his assertions of popular rights, so cautious as to reduce them almost to nothing. It is not till we come to Rousseau that the second form of the contract theory is stated in its purest and most logical form.