This work was in the course of preparation for the press when its great Author's earthly labors were suddenly closed. He had, up to the latest period of his existence, been accustomed to record the desultory thoughts which occurred to his mind on the important subject of which the volume treats; and I had the advantage, in my intimate communion with him, of seeking such guidance from him as was necessary for the understanding and arrangement of the mass of undigested fragments which he from time to time placed in my hands.