If two ideas form an exact logical antithesis, that is to say if the one is the complement of the other, then, in fact, each one is implied in the other; and when the limited power of our mind is insufficient to apprehend both at once, and, by the mere antithesis, to recognise in the one perfect conception the totality of the other also, still, at all events, the one always throws on the other a strong, and in many parts a sufficient light Thus we think the first chapter on the defence throws a sufficient light on all the points of the attack which it touches upon. But it is not so throughout in respect of every point; the train of thought could nowhere be carried to a finality; it is, therefore, natural that where the opposition of ideas does not lie so immediately at the root of the conception as in the first chapters, all that can be said about the attack does not follow directly from what has been said on the defence. An alteration of our point of view brings us nearer to the subject, and it is natural for us to observe, at this closer point of view, that which escaped observation at our former standpoint. What is thus perceived will, therefore, be the complement of our former train of thought; and it will not unfrequently happen that what is said on the attack will throw a new light on the defence.
In treating of the attack we shall, of course, very frequently have the same subjects before us with which our attention has been occupied in the defence. But we have no intention, nor would it be consistent with the nature of the thing, to adopt the usual plan of works on engineering, and in treating of the attack, to circumvent or upset all that we have found of positive value in the defence, by showing that against every means of defence, there is an infallible method of attack. The defence has its strong