It is clear that the history, in order to rise above a chronicle, must inquire into the causes which have made freedom of inquiry develop into unbelief. The causes have usually been regarded by theologians to be of two kinds, viz. either superhuman or human; and, if of the latter kind, to be either moral or intellectual. Bishop Van Mildert, in his History of Infidelity, restricted himself entirely to the former.(12) Holding strongly that the existence of evil in the world was attributable, not only indirectly and originally, but directly and perpetually, to the operation of the evil spirit, he regarded every form of heresy and unbelief to be the attempt of an invisible evil agent to thwart the truth of God; and viewed the history of infidelity as the study of the results of the operation of this cause in destroying the kingdom of righteousness. Such a view invests human life and history with a very solemn character, and is not without practical value; but it will be obvious that an analysis of this kind must be strictly theological, and removes the inquiry from the province of human science. Even when completed, it leaves unexplored the whole field in which such an evil principle operates, and the agencies which he employs as his instruments. The majority of writers on unbelief accordingly have treated the subject from a less elevated point of view, and have limited their inquiry to the sphere of the operation of human causes, the _media axiomata_ as it were,(13) which express the motives and agencies which have been manifested on the theatre of the world, and visible in actual history. It will be clear that within this sphere the causes are specially of two kinds; viz. those which have their source in the will, and arise from the antagonism of feeling, which wishes revelation untrue, and those which manifest themselves in the intellect, and are exhibited under the form of difficulties which beset the mind, or doubts which mislead it, in respect to the evidence on which revelation reposes. The former, it may be feared, are generally the ground of unbelief; the latter the basis of doubt. Christian writers, in the wish to refer unbelief to the source of efficient causation in the human will, with a view of enforcing on the doubter the moral lesson of responsibility, have generally restricted themselves to the former of these two classes; and by doing so have omitted to explore the interesting field of inquiry presented in the natural history of the variety of forms assumed by scepticism, and their relation to the general causes which have operated in particular ages:—a subject most important, if the intellectual antecedents thus discovered be regarded as causes of doubt; and not less interesting, if, instead of being causes, they are merely considered to be instruments and conditions made use of by the emotional powers.