The celebrated David Hume was one of those geographers of human reason who believe that they have given a sufficient answer to all such questions by declaring them to lie beyond the horizon of our knowledge- a horizon which, however, Hume was unable to determine.His attention especially was directed to the principle of causality; and he remarked with perfect justice that the truth of this principle, and even the objective validity of the conception of a cause, was not commonly based upon clear insight, that is, upon a priori cognition.
Hence he concluded that this law does not derive its authority from its universality and necessity, but merely from its general applicability in the course of experience, and a kind of subjective necessity thence arising, which he termed habit.From the inability of reason to establish this principle as a necessary law for the acquisition of all experience, he inferred the nullity of all the attempts of reason to pass the region of the empirical.
This procedure of subjecting the facta of reason to examination, and, if necessary, to disapproval, may be termed the censura of reason.This censura must inevitably lead us to doubts regarding all transcendent employment of principles.But this is only the second step in our inquiry.The first step in regard to the subjects of pure reason, and which marks the infancy of that faculty, is that of dogmatism.The second, which we have just mentioned, is that of scepticism, and it gives evidence that our judgement has been improved by experience.But a third step is necessary- indicative of the maturity and manhood of the judgement, which now lays a firm foundation upon universal and necessary principles.This is the period of criticism, in which we do not examine the facta of reason, but reason itself, in the whole extent of its powers, and in regard to its capability of a priori cognition; and thus we determine not merely the empirical and ever-shifting bounds of our knowledge, but its necessary and eternal limits.We demonstrate from indubitable principles, not merely our ignorance in respect to this or that subject, but in regard to all possible questions of a certain class.Thus scepticism is a resting place for reason, in which it may reflect on its dogmatical wanderings and gain some knowledge of the region in which it happens to be, that it may pursue its way with greater certainty; but it cannot be its permanent dwelling-place.It must take up its abode only in the region of complete certitude, whether this relates to the cognition of objects themselves, or to the limits which bound all our cognition.