Thus, instances occur now and then, when, in consequence of the urgency of some fashionable superstition or popular delusion, some eminent scientific authority is provoked to come forward, and to set the world right by his “ipse dixit.” He, indeed, himself knows very well what he is about; he has a right to speak, and his reasonings and conclusions are sufficient, not only for his own, but for general assent, and, it may be, are as simply true and impregnable, as they are authoritative; but an intelligent hold on the matter in dispute, such as he has himself, cannot be expected in the case of men in general. They, nevertheless, one and all, repeat and retail his arguments, as suddenly as if they had not to study them, as heartily as if they understood them, changing round and becoming as strong antagonists of the error which their master has exposed, as if they had never been its advocates. If their word is to be taken, it is not simply his authority that moves them, which would be sensible enough and suitable in them, both apprehension and assent being in that case grounded on the maxim “Cuique in arte suâ credendum,” but so far forth as they disown this motive, and claim to judge in a scientific question of the worth of arguments which require some real knowledge, they are little better, not of course in a very serious matter, than pretenders and formalists. Not only Authority, but Inference also may impose on us assents which in themselves are little better than assertions, and which, so far as they are assents, can only be notional assents, as being assents, not to the propositions inferred, but to the truth of those propositions. For instance, it can be proved by irrefragable calculations, that the stars are not less than billions of miles distant from the earth; and the process of calculation, upon which such statements are made, is not so difficult as to require authority to secure our acceptance of both it and of them; yet who can say that he has any real, nay, any notional apprehension of a billion or a trillion? We can, indeed, have some notion of it, if we analyze it into its factors, if we compare it with other numbers, or if we illustrate it by analogies or by its implications; but I am speaking of the vast number in itself. We cannot assent to a proposition of which it is the predicate; we can but assent to the truth of it.