(2) With sceptical tendencies in Scholasticism, in the middle ages (1100-1400). (p. 8.) (3) With literature, at the Renaissance, in Italy (1400-1625). (p. 9.) (4) With modern philosophy in three forms (p. 11): viz. English Deism in the seventeenth and eighteenth centuries (p. 11); French Infidelity in the eighteenth century; German Rationalism in the eighteenth and nineteenth. Proposal to study the natural as well as literary history of these forms of doubt.—The investigation separated from inquiries into heresy as distinct from scepticism. (p. 13.) The causes, seen to act according to the law just described, which make free thought develope into unbelief, stated to be twofold. (p. 13.) 1. Emotional causes.—Necessity for showing the relation of the intellectual causes to the emotional, both per se, and because the idea of a history of thought, together with the comparative rarity of the process here undertaken, implies the restriction of the attention mainly to the intellectual. (p. 13.) Influence of the emotional causes shown, both from psychology and from the analysis of the nature of the evidence offered in religion (pp. 14, 15).—Historical illustrations of their influence. (pp. 15-17.) Other instances where the doubt is in origin purely intellectual (p. 17), but where nevertheless opportunity is seen for the latent operation of the emotional. (p. 18.) Explanation how far religious doubt is sin. (pp. 19, 20.) 2. Intellectual causes, which are the chief subject of these lectures; the conjoint influence however of the emotional being always presupposed. The intellectual causes shown to be (p. 20): (α) the new material of knowledge which arises from the advance of the various sciences; viz. Criticism; Physical, Moral, and Ontological science. (p. 21.) (β) the various metaphysical tests of truth or grounds of certitude employed. (p. 22.) An illustration of the meaning (pp. 22, 23), drawn from literature, in a brief comparison of the types of thought shown in Milton, Pope, and Tennyson. Statement of the exact position of this inquiry in the subdivisions of metaphysical science (pp. 24, 25), and detailed explanation of the advantages and disadvantages of applying to religion the tests of Sense, subjective Forms of Thought, Intuition, and Feeling, respectively; as the standard of appeal. (pp. 25-32.) Advantage of a biographic mode of treatment in the investigation of the operation of these causes in the history of doubt. (pp. 32-34.) Statement of the utility of the inquiry: (1) Intellectually, (α) in a didactic and polemical point of view, in that it refers the origin of the intellectual elements in error to false philosophy and faulty modes of judging, and thus refutes error by analysing it into the causes which produce it; and also (β) in an indirect contribution to the Christian evidences by the historic study of former contests. (p. 36.)