A history of free thought seems to point especially to the study of the latter class. A biographical history of free thinkers would imply the former; the investigation of the moral history of the individuals, the play of their will and feelings and character; but the history of free thought points to that which has been the product of their characters, the doctrines which they have taught. Science however no less than piety would decline entirely to separate the two;(14) piety, because, though admitting the possibility that a judgment may be formed in the abstract on free thought, it would feel itself constantly drawn into the inquiry of the moral responsibility of the freethinker in judging of the concrete cases;—science, because, even in an intellectual point of view, the analysis of a work of art is defective if it be studied apart from the personality of the mental and moral character of the artist who produces it. If even the inquiry be restricted to the analysis of intellectual causes, a biographic treatment of the subject, which would allow for the existence of the emotional, would be requisite.(15) The province of the following work accordingly is, the examination of this neglected branch in the analysis of unbelief. While admitting most fully and unhesitatingly the operation of emotional causes, and the absolute necessity, scientific as well as practical, of allowing for their operation, it is proposed to analyse the forms of doubt or unbelief in reference mainly to the intellectual element which has entered into them, and the discovery of the intellectual causes which have produced or modified them. Thus the history, while not ceasing to belong to church history, becomes also a chapter in the history of philosophy, a page in the history of the human mind. The enumeration of the causes into which the intellectual elements of doubt are resolvable, is furnished in the text of the first Lecture.(16) If the nature of some of them be obscure, and the reader be unaccustomed to the philosophical study necessary for fully understanding them; information must be sought in the books to which references are elsewhere given, as the subject is too large to be developed in the limited space of this Preface.