In both forms, it has existed, either continuously or in ever-recurring cycles, from the earliest dawn of speculative inquiry; and while it has seemed to retard or arrest the progress of human knowledge, it has really been overruled as a means of quickening the intellectual powers, and imparting at once greater precision and comprehensiveness to the matured results of Science.
Theoretical Skepticism may be divided into _three_ distinct branches: First, Universal or Philosophical Skepticism, which professes to deny, or rather to doubt the certainty of all human knowledge; secondly, Partial or Religious Skepticism, which admits the possible certitude of human knowledge in other respects, but holds that religious truth is either altogether inaccessible to our faculties, or that it is not supported by sufficient evidence; thirdly, a mongrel system, which combines Philosophic Doubt with Ecclesiastical Dogmatism, and which may be aptly characterized as the Skeptico-Dogmatic theory.[238] We agree with Dr.Reid in thinking that Universal Skepticism is unanswerable _by argument_, and can only be effectively met by an _appeal to consciousness_.[239] It might be shown, indeed, that in so far as it assumes, however slightly, the aspect of a positive or dogmatic system, it is self-contradictory and absurd; it might also be shown that doubt itself implies thought, and thought existence or reality: but the ultimate appeal must be to the facts of human consciousness, and the laws of thought which operate in every human breast.

And when such an appeal is made, we can have no anxiety in regard to the result, nor any apprehension that philosophical skepticism can ever become the prevailing creed of the popular mind.

There is a risk, however, of danger arising from a different source; it may not be always remembered that the theory of Skepticism must be universal to be either consistent or consequent; and hence it may be _partially applied_ to some truths, while it is practically abandoned in regard to other truths, which are neither more certain nor less liable to objection than the former.

Thus the skeptical difficulties which have been raised against the doctrines of Ontology are of such a kind that if they have any validity or force, they bear as strongly against the reality of an external world and the existence of our fellow-men, as against the doctrine which affirms the being of God: yet many will be found urging them against the latter doctrine, who do not profess to have any doubt in regard to the two former; and it is of paramount importance to show that this is a partial and therefore unfair application of their own principles, and that they cannot consistently admit the one without also admitting the other.
Atheism, in its skeptical form, must either be a mere _sense of doubt_ in regard to the sufficiency of the evidence in favor of the being and perfections of God; or _a speculative system_, which attempts to justify that doubt by some theory of philosophical skepticism, either partial or universal.

In the _latter_ case, it may be best dealt with by showing that it affects the certainty of our common knowledge, not less than that of our religious belief, and that we cannot consistently reject Theology, and yet retain our convictions on other cognate subjects of thought.