XXIX. I have always wondered, and do not yet cease to wonder, at seeing modern philosophers attack scepticism as a physical error, and am much astonished, to find them condemn it in that sense as an error, dangerous to the dogmas of faith. Nor can I comprehend, how such a charge can arise, otherwise, than from gross ignorance or malicious spite; unless it is applied to some sceptic, who from explaining himself loosely, may have given occasion for such an imputation. XXX. The system of physical scepticism, asserts, that physical and natural things, do not admit of demonstration or scientific certainty, but that they are all matter of opinion; and consequently, that we should not call natural philosophy a science, because strictly speaking it is not one, but rather a habit, or acquired facility, of reasoning with probability upon natural things. We shall understand science in this place, in the sense which Aristotle and the schoolmen understand it, who define it, _to be an evident knowledge of the effect by the cause_. But in our definitions, we shall not exclude experimental certainty, or certain knowledge, acquired by the experience we have had, and the observations we have made, of physical matters; but we shall rather maintain it, as the only method by which we can arrive at attaining truth; although I doubt whether we shall ever attain by it, a discovery of the internal, or intimate nature of things. XXXI. Neither will we deny, that with respect to physical objects, many propositions may be advanced or supported, which are deducible with infallible certainty from metaphysical principles: as for example from this, _the whole is greater than its part_, is evidently inferred, that a man is bigger than his head; and from this, _being is the effect of operation_, is inferred, that my father existed when he begot me. But these, and other innumerable demonstrations of this sort, afford no physical knowledge whatever; because they do not manifest in any degree, either great or small, the nature of the very entities which are the objects of them. Why do I say manifest the nature of the entities? They do not even display to the understanding, a single truth, which may not be comprehended without their help, by the most rustic man upon earth. So that the syllogistical conclusions upon infallible truths, which the school philosophers so much boast of, do nothing more, than explain by circumlocution and in terms of art, the very things, which may be immediately comprehended, and naturally explained, by any rational man who never studied. Further, how can they call by the name of demonstrations, things which demonstrate nothing? that is, they manifest nothing, but what was manifest without their assistance. A logician will say, thinking he says something to the purpose, that he knows by means of the art of demonstration, that which he could not know artificially without this aid. But I answer, that this artificial knowledge is