A waking man, being under the necessity of having some ideas constantly in his mind, is not at liberty to think or not to think; no more than he is at liberty, whether his body shall touch any other or no, but whether he will remove his contemplation from one idea to another is many times in his choice; and then he is, in respect of his ideas, as much at liberty as he is in respect of bodies he rests on; he can at pleasure remove himself from one to another.