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.
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.