A man that walks is at liberty also, not because he walks or moves; but because he can stand still if he wills it. But if a man sitting still has not a power to remove himself, he is not at liberty
 John Locke, An Essay Concerning Human Understanding (1689). copy citation

Context

“This, then, is evident, That A MAN IS NOT AT LIBERTY TO WILL, OR NOT TO WILL, ANYTHING IN HIS POWER THAT HE ONCE CONSIDERS OF: liberty consisting in a power to act or to forbear acting, and in that only. For a man that sits still is said yet to be at liberty; because he can walk if he wills it. A man that walks is at liberty also, not because he walks or moves; but because he can stand still if he wills it. But if a man sitting still has not a power to remove himself, he is not at liberty; so likewise a man falling down a precipice, though in motion, is not at liberty, because he cannot stop that motion if he would. This being so, it is plain that a man that is walking, to whom it is proposed to give off walking, is not at liberty, whether he will determine himself to walk, or give off walking or not:” source