BUT I am wandering from what I was intending to do, that is, make plainer than perhaps appears in the previous chapters, some of the peculiar requirements of the science of piloting.

First of all, there is one faculty which a pilot must incessantly cultivate until he has brought it to absolute perfection.

Nothing short of perfection will do.
That faculty is memory.

He cannot stop with merely thinking a thing is so and so; he must know it; for this is eminently one of the 'exact' sciences.

With what scorn a pilot was looked upon, in the old times, if he ever ventured to deal in that feeble phrase 'I think,' instead of the vigorous one 'I know!' One cannot easily realize what a tremendous thing it is to know every trivial detail of twelve hundred miles of river and know it with absolute exactness.