THE investigation of truth would be far easier than it is, if truth lay at the end of a straight line, so that we could advance to it step by step, making sure of each step before we passed on to the next. As a matter of fact, truth is like an arch, in which every part must be carefully articulated to the rest; and until the last brick is fitted into its place, we cannot be said to have anything of the arch at all. We may single out one piece as the keystone; but for practical purposes, every piece is the keystone to the rest. So with our own argument in the preceding chapters. We have not advanced—we could not advance—step by step; we have been laying bricks, for no one of which could we be completely ready, till every other brick had been put into its proper place.