The word “free thought” is now commonly used, at least in foreign literature(6), to express the result of the revolt of the mind against the pressure of external authority in any department of life or speculation. Information concerning the history of the term is given elsewhere.(7) It will be sufficient now to state, that the cognate term, _free thinking_, was appropriated by Collins early in the last century(8) to express Deism. It differs from the modern term _free thought_, both in being restricted to religion, and in conveying the idea rather of the method than of its result, the freedom of the mode of inquiry rather than the character of the conclusions attained; but the same fundamental idea of independence and freedom from authority is implied in the modern term. Within the sphere of its application to the Christian religion, free thought is generally used to denote three different systems; viz. Protestantism, scepticism, and unbelief. Its application to the first of these is unfair.(9) It is true that all three agree in resisting the dogmatism of any earthly authority; but Protestantism reposes implicitly on what it believes to be the divine authority of the inspired writers of the books of holy scripture; whereas the other two forms acknowledge no authority external to the mind, no communication superior to reason and science. Thus, though Protestantism by its attitude of independence seems similar to the other two systems, it is really separated by a difference of kind, and not merely of degree.(10) The present history is restricted accordingly to the treatment of the two latter species of free thought,—the resistance of the human mind to the Christian religion as communicated through revelation, either in part or in whole, neither the scepticism which disintegrates it, or the unbelief which rejects it: the former directing itself especially against Christianity, the latter against the idea of revelation, or even of the supernatural generally. An analogous reason to that which excludes the history of Protestantism, excludes also that of the opposition made to Christianity by heresy, and by rival religions:(11) inasmuch as they repose on authorities, however false, and do not profess to resort to an unassisted study of nature and truth. This account of the province included under free thought will prepare the way for the explanation of the mode in which the subject is treated.