n _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.