This view, that doubt might be refuted by the historic analysis of its operation, by laying bare the antecedent grounds which had produced it, will explain why the author was led to believe that a chapter of mental and moral physiology might be useful, which would not merely carry out the anatomy of actual forms of disease, but discover their origin by the study of the preceding natural history of the patients. These remarks will perhaps suffice for explaining the object which was proposed in writing this history; and may justify the hope that this work, thus adapted to the wants of the time, may offer such a contribution to the subject of the Christian evidences, as not only to possess an intellectual value, but to coincide with the purpose contemplated by the founder of the Lectures. It remains to state the sources which have been used for the literary materials of the history. Though they are sufficiently indicated in the notes, a general description of them may be useful. They may be distributed under four classes; 1. The histories which have been professedly devoted to the subject. 2. The notices of the history of unbelief in general histories of the church or of literature. 3. (Which ought indeed to rank first in importance;) the original authorities for the facts, i.e. the works of the sceptical writers themselves; or of the contemporary authors who have refuted them. 4. The monographs, which treat of particular writers, ages, or schools, of sceptical thought. In approaching the subject, a student would probably commence with the first two classes; and after having thus acquired for himself a _carte du pays_, would then explore it in detail by the aid of the third and fourth. 1. The works which have professedly treated of the history of infidelity, as a whole, are not of great importance. One of the earliest was the _Historia Univ. Atheismi_, 1725, of Reimannus; and the _De Atheismo_, 1737, of Buddeus. (An explanation of the word _Atheism_, as employed by them, is given in Note 21. p. 413.) hey furnish, as the name implies, a history of scepticism, as well as of sceptics; yet, though the labours of such diligent and learned men can never be useless, they afford little information now available. Their date also necessarily precluded them from knowing the more recent forms of unbelief. Perhaps under this head we ought also to name the chapters on polemical theology in the great works of bibliography of the German scholars of the same time, such as Pfaff (_Hist. Litt. Thol._); Buddeus (_Isagoge_); Fabricius (_Delectus Argum._); Walch’s (_Biblical Theol. Select._); which contain lists of sceptical works, either directly, or indirectly by naming the apologists who have answered them. The references to these works will be found in Note 39. p. 436.