The chief portion of the present lecture will be devoted to explain the mode of applying the plan just indicated; more especially to develop the second of these three branches, by stating the law which has marked the struggle of free thought with Christianity, and illustrating the intellectual causes which have been manifested in it. In searching for such a law, or such causes, we ought not to forget that, if we wished to lay a sound basis for generalization, it would be necessary not to restrict our attention to the history of Christianity, but to institute a comparative study of religions, ethnic or revealed, in order to trace the action of reason in the collective religious history of the race. Whether the religions of nature be regarded as the distortion of primitive traditions, or as the spontaneous creation of the religious faculties, the agreement or contrast suggested by a comparison of them with the Hebrew and Christian religions, which are preternaturally revealed, is most important as a means of discovering the universal laws of the human mind; the exceptional character which belongs to the latter member of the comparison increasing rather than diminishing the value of the study. All alike are adjusted, the one class naturally and accidentally, the other designedly and supernaturally, to the religious elements of human nature. All have a subjective existence as aspirations of the heart, an objective as institutions, and a history which is connected with the revolutions of literature and society. (2) Comparative observation of this kind gives some approach to the exactness of experiment; for we watch providence as it were executing an experiment for our information, which exhibits the operations of the same law under altered circumstances. If, for example, we should find that Christianity was the only religion, the history of which presented a struggle of reason against authority, we should pronounce that there must be peculiar elements in it which arouse the special opposition; or if the phenomenon be seen to be common to all creeds, but to vary in intensity with the activity of thought and progress of knowledge, this discovery would suggest to us the existence of a law of the human mind.