We have seen that both the search for causes and the examination of means lead into the field of theory; that is, into the field of universal truth, which does not proceed solely from the case immediately under examination. If there is a theory which can be used, then the critical consideration will appeal to the proofs there afforded, and the examination may there stop. But where no such theoretical truth is to be found, the inquiry must be pushed up to the original elements. If this necessity occurs often, it must lead the historian (according to a common expression) into a labyrinth of details. He then has his hands full, and it is impossible for him to stop to give the requisite attention everywhere; the consequence is, that in order to set bounds to his investigation, he adopts some arbitrary assumptions which, if they do not appear so to him, do so to others, as they are not evident in themselves or capable of proof.

A sound theory is therefore an essential foundation for criticism, and it is impossible for it, without the assistance of a sensible theory, to attain to that point at which it commences chiefly to be instructive, that is, where it becomes demonstration, both convincing and sans réeplique.

But it would be a visionary hope to believe in the possibility of a theory applicable to every abstract truth, leaving nothing for criticism to do but to place the case under its appropriate law: it would be ridiculous pedantry to lay down as a rule for criticism that it must always halt and turn round on reaching the boundaries of sacred theory. The same spirit of analytical inquiry which is the origin of theory must also guide the critic in his work; and it can and must therefore happen that he strays beyond the boundaries of the province of theory and elucidates those points with which he is more particularly concerned. It is more likely, on the contrary, that criticism would completely fail in its object if it degenerated into a mechanical application of theory. All positive results of theoretical inquiry, all principles, rules, and methods, are the more wanting in generality and positive truth the more they become positive doctrine. They exist to offer themselves for use as required, and it must always be left for judgement to decide whether they are suitable or not. Such results of theory must never be used in criticism as rules or norms for a standard, but in the same way as the person acting should use them, that is, merely as aids to judgement. If it is an acknowledged principle in tactics that in the usual order of battle cavalry should be placed behind infantry, not in line with it, still it would be folly on this account to condemn every deviation from this principle. Criticism must investigate the grounds of the deviation, and it is only in case these are insufficient that it has a right to appeal to principles laid down in theory. If it is further established in theory that a divided attack diminishes the probability of success, still it would be just as unreasonable, whenever there is a divided attack and an unsuccessful issue, to regard the latter as the result of the former, without further investigation into the connection between the two, as where a divided attack is successful to infer from it the fallacy of that theoretical principle. The spirit of investigation which belongs to criticism cannot allow either. Criticism therefore supports itself chiefly on the results of the analytical investigation of theory; what has been made out and determined by theory does not require to be demonstrated over again by criticism, and it is so determined by theory that criticism may find it ready demonstrated.

This office of criticism, of examining the effect produced by certain causes, and whether a means applied has answered its object, will be easy enough if cause and effect, means and end, are all near together.

If an army is surprised, and therefore cannot make a regular and intelligent use of its powers and resources, then the effect of the surprise is not doubtful. If theory has determined that in a battle the convergent form of attack is calculated to produce greater but less certain results, then the question is whether he who employs that convergent form had in view chiefly that greatness of result as his object; if so, the proper means were chosen. But if by this form he intended to make the result more certain, and that expectation was founded not on some exceptional circumstances (in this case), but on the general nature of the convergent form, as has happened a hundred times, then he mistook the nature of the means and committed an error.

Here the work of military investigation and criticism is easy, and it will always be so when confined to the immediate effects and objects. This can be done quite at option, if we abstract the connection of the parts with the whole, and only look at things in that relation.