It is one peculiar advantage belonging to the philosophical investigation of the history of thought, that even the odious becomes valuable as an object of study, the pathology of the soul as well as its normal action. Philosophy takes cognisance of error as well as of truth, inasmuch as it derives materials from both for discovering a theory of the grounds of belief and disbelief. Hence it follows that the study of the natural history of doubt combined with the literary, if it be the means of affording an explanation of a large class of facts relating to the religious history of man and the sphere of the remedial operations of Christ’s church, will have a practical value as well as speculative. Such an inquiry, if it be directed, as in the present lectures, to the analysis of the intellectual rather than the emotional element of unbelief, as being that which has been less generally and less fully explored, will require to be supplemented by a constant reference to the intermixture of the other element, and the consequent necessity of taking account of the latter in estimating the whole phenomenon of doubt. But within its own sphere it will have a practical and polemical value, if the course of the investigation shall show that the various forms of unbelief, when studied from the intellectual side, are corollaries from certain metaphysical or critical systems. The analysis itself will have indirectly the force of an argument. The discovery of the causes of a disease contains the germ of the cure. Error is refuted when it is referred to the causes which produce it. Nor will the practical value of the inquiry be restricted to its use as a page in the spiritual history of the human mind, but will belong to it also as a chapter in the history of the church. For even if in the study of the contest our attention be almost wholly restricted to the movements of one of the two belligerents, and only occasionally directed to the evidences on which the faith of the church in various crises reposed, and by which it tried to repel the invader, yet the knowledge of the scheme of attack cannot fail to be a valuable accompaniment to the study of the defence.(117) Thus the natural history of doubt, viewed as a chapter of human history, like the chapter of physiology which studies a disease, will point indirectly to the cure, or at least to the mode of avoiding the causes which induce the disease; while the literary history of it, viewed as a chapter of church history, will contribute the results of experience to train the Christian combatant.