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 re’plique.

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 judgment 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 judgment.  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