Such examples, indeed, are not restricted to Christian history, but are general facts of the history of the human mind. The fifth century B.C. was such an epoch in Greece;(1020) when various causes, social and intellectual, created a sudden awakening of the human mind to reconsider its old beliefs, and find a home for the new views of nature and of the world which were opening. The free thought of the Sophists was the scepticism of doubt, of distrust; the proposal to surrender, to destroy the old: the free thought of Socrates was the scepticism of inquiry, the attempt to reconsider first principles, to rebuild truth anew. In all such moments, investigation is indirectly the means of stimulating knowledge. The history of the progress of it, in reference to the difficulties which have beset the Christian church, shows us that the epochs of doubt have not generally been produced by unbelief taking the initiative in attacking old truths without some fresh stimulus, and repeating old objections so as to exhibit perpetually recurring cycles of unbelief. We have rather seen that doubt is reawakened by the introduction of new forms of knowledge; and though old doubts recur, yet that they come arrayed in a new garb, suggested by different motives, deduced from fresh premises, and accompanied by doubts of a new kind before unknown. In a practical point of view, frequently they may be thought not to differ widely in appearance from old ones, and to present similar effects as well as forms; but in a scientific one, they ought not to be confounded, inasmuch as they do not present identity of cause. There has been a slow but real progress in knowledge, and a slow but real change in the modes of applying it to Christian religion. The effect of the defence offered for Christianity is equally powerful in leaving its impress on subsequent doubt, as the progress of knowledge is in suggesting novelty of form. The sphere is narrowed, or the direction changed. If thought seems to have come round in its revolution to the same spot in its orbit, it will be found to be moving not on a circle, but on a spiral; slowly but surely approaching a little nearer to the great central truth, toward which it is unconsciously attracted.