Thirdly, the testing of the means employed.  This is criticism, properly speaking, in which praise and censure is contained.  This is where theory helps history, or rather, the teaching to be derived from it.

In these two last strictly critical parts of historical study, all depends on tracing things to their primary elements, that is to say, up to undoubted truths, and not, as is so often done, resting half-way, that is, on some arbitrary assumption or supposition.

As respects the tracing of effect to cause, that is often attended with the insuperable difficulty that the real causes are not known.  In none of the relations of life does this so frequently happen as in War, where events are seldom fully known, and still less motives, as the latter have been, perhaps purposely, concealed by the chief actor, or have been of such a transient and accidental character that they have been lost for history.  For this reason critical narration must generally proceed hand in hand with historical investigation, and still such a want of connection between cause and effect will often present itself, that it does not seem justifiable to consider effects as the necessary results of known causes.  Here, therefore must occur, that is, historical results which cannot be made use of for teaching.  All that theory can demand is that the investigation should be rigidly conducted up to that point, and there leave off without drawing conclusions.  A real evil springs up only if the known is made perforce to suffice as an explanation of effects, and thus a false importance is ascribed to it.

Besides this difficulty, critical inquiry also meets with another great and intrinsic one, which is that the progress of events in War seldom proceeds from one simple cause, but from several in common, and that it therefore is not sufficient to follow up a series of events to their origin in a candid and impartial spirit, but that it is then also necessary to apportion to each contributing cause its due weight.  This leads, therefore, to a closer investigation of their nature, and thus a critical investigation may lead into what is the proper field of theory.

The critical consideration, that is, the testing of the means, leads to the question, Which are the effects peculiar to the means applied, and whether these effects were comprehended in the plans of the person directing?

The effects peculiar to the means lead to the investigation of their nature, and thus again into the field of theory.

We have already seen that in criticism all depends upon attaining to positive truth; therefore, that we must not stop at arbitrary propositions which are not allowed by others, and to which other perhaps equally arbitrary assertions may again be opposed, so that there is no end to pros and cons; the whole is without result, and therefore without instruction.