There are indications that on at least one occasion he subjected his beliefs to a careful scrutiny, and, referring to this later, he spoke of himself as one who, in the words of the Roman poet, had been "much tossed about on land and on the deep ere he could build a city." This, coming from one who was habitually reticent about his religious experiences, may be held as proving that there was no want of rigour in the process, no withholding of any part of the structure from the strain. But that that structure ever gave way, that it ever came tumbling down in ruins about him so that it had to be built again on new foundations, there is no evidence to show. The "_aliquid inconcussum_" appears to have remained with him all through the experience. This seems clear from a passage in a letter written in 1848 to his brother David, then a student in Sir William Hamilton's class, in which he says; "I never found my religious susceptibilities injured by metaphysical speculations. Whether this was a singular felicity I do not know, but I have heard others complain."[13] [Footnote 13: _Life and Letters_, p. 295.] This, taken in conjunction with the passage quoted above from Clark's Life, in which it is hard to believe that he is not speaking of himself, seems decisive enough, and in a mind of such speculative grasp and activity it is remarkable. "Right down through the storm-zone of the nineteenth century," writes one who knew him well, "he comes untroubled by the force of the '_aliquid inconcussum_.' Edinburgh, Germany, Berwick; Hamilton, Kant, Hegel, Strauss, Renan, it is all the same. The cause seems to me luminously plain. Saints are never doubters. His religious intuitions were so deep and clear that he was able always to find his way by their aid. They gave him his independent certainty, his '_aliquid inconcussum_.'" His influence on the religious life of his time was largely due to the spiritual faculty in him that is here referred to. He was the power he was, not so much because of his intellectual strength as because of his character,--because he was "a great Christian." But in this respect he had the defects of his qualities; and it is open to question whether he ever truly appreciated the formidable character of modern doubt, just because he himself had never had full experience of its power, because the iron of it had never really entered into his soul.