more is necessary than to appeal to his own experience and belief, which is here as valid and irresistible as in regard to the existence of God, of moral distinctions, or of the material world.  He has no reason to trust the one class of beliefs which he has not, to trust the other....  To minds thus favoured, this forms a point d’appui which can never be overturned—­an aliquid inconcussum corresponding to the ‘cogito ergo sum’ of Descartes.  Their faith bears its own signature, and they have only to look within to discover its authenticity.  Philosophy must be guided by experience, and must rank the characters inscribed on the soul by grace at least as sacred as those inscribed by nature.  Such persons need not that any man should teach them, for they have an unction from the Holy One; and to them applies the highest of all congratulations:  ’Blessed art thou; for flesh and blood hath not revealed it unto thee, but My Father which is in heaven.’"[12]

[Footnote 12:  Fragments of College and Pastoral Life, pp. 38-40.]

These words contain the true explanation of Cairns’s life.  There was in it the “aliquid inconcussum”—­the “unshaken somewhat”—­which made him independent of other arguments, and which kept him untouched by all the intellectual attacks on Christianity.  Other people who had not this inward testimony, or who, having it, could not regard it as unshaken by the assaults of infidelity, he could argue with and seek to meet them on their own intellectual ground; but for himself, any victories gained here were superfluous, any defects left him unmoved.  Was it always so with him?  Or was there ever a time when he was carried off his feet and had to struggle for dear life for his Christian faith amid the dark waters of doubt?

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.]