If this verdict is confessedly true with regard to many observations which we make on things immediately present to our senses, much more likely is it to be true with respect to conclusions which are not "the immediate results of sensible impressions," but are merely deduced by a process of reasoning from such impressions. And if the direct evidence of our senses is to be received with a prudent reserve, because of this possibility of error, even when we have no evidence of an opposing character, still more necessary is the exercise of caution in judging of facts assumed to have occurred at a period far removed from our own experience, and which stand in contradiction (at least apparent, _prima facie_, contradiction) to credible historic testimony. Nay, the caveat acquires a greatly intensified force, when the testimony with which the assumed facts are, or seem to be, at variance, is no less a testimony than His who ordained the "facts," who made the objects of investigation; the testimony of the Creator of all things; the testimony of Him who is, from eternity to eternity, "[Greek: HO APSEUDES THEOS]"! I hope I shall not be deemed censorious in stating my fear that those who cultivate the physical sciences are not always sufficiently mindful of the "_Humanum est errare_." What we have investigated with no little labour and patience, what we have seen with our eyes many many times, in many aspects, and under many circumstances, we naturally believe firmly; and we are very prone to attach the same assurance of certainty to the inferences we have, _bona fide_, and with scrupulous care to eliminate error, deduced from our observations, as to the observations themselves; and we are apt to forget that some element of error may have crept into our actual investigations, and still more probably into our deductions. Even if our observations be so simple, so patent, so numerous, as _almost_ to preclude the possibility of mistake in them, and our process of reasoning from them be without a flaw, still we may have overlooked a principle, which, though perhaps not very obvious, ought to enter into the investigation, and which, if recognised, would greatly modify our conclusions. In this volume I venture to suggest such a principle to the consideration of geologists. It will not be denied that Geology is a science that stands peculiarly in need of being cultivated with that salutary self-distrust that I have above alluded to. Though a strong and healthy child, it is as yet but an infant. The objects on which its senses have been exercised, its [Greek: ta blepomena], are indeed plain enough and numerous enough, when once discovered; but the inferences drawn from them, its [Greek: bebaia], find their sphere in the most venerably remote antiquity,--an antiquity mensurable not by years or centuries, but by _secula seculorum_. And the dicta, which its votaries rest on as certitudes, are at variance with the simple literal sense of the words of God.