Chapter 29.—To What Extent Natural Science is an Exegetical Aid. 45.  There is also a species of narrative resembling description, in which not a past but an existing state of things is made known to those who are ignorant of it.  To this species belongs all that has been written about the situation of places, and the nature of animals, trees, herbs, stones, and other bodies.  And of this species I have treated above, and have shown that this kind of knowledge is serviceable in solving the difficulties of Scripture, not that these objects are to be used conformably to certain signs as nostrums or the instruments of superstition; for that kind of knowledge I have already set aside as distinct from the lawful and free kind now spoken of.  For it is one thing to say:  If you bruise down this herb and drink it, it will remove the pain from your stomach; and another to say:  If you hang this herb round your neck, it will remove the pain from your stomach.  In the former case the wholesome mixture is approved of, in the latter the superstitious charm is condemned; although indeed, where incantations and invocations and marks are not used, it is frequently doubtful whether the thing that is tied or fixed in any way to the body to cure it, acts by a natural virtue, in which case it may be freely used; or acts by a sort of charm, in which case it becomes the Christian to avoid it the more carefully, the more efficacious it may seem to be.  But when the reason why a thing is of virtue does not appear, the intention with which it is used is of great importance, at least in healing or in tempering bodies, whether in medicine or in agriculture. 46.  The knowledge of the stars, again, is not a matter of narration, but of description.  Very few of these, however, are mentioned in Scripture.  And as the course of the moon, which is regularly employed in reference to celebrating the anniversary of our Lord’s passion, is known to most people; so the rising and setting and other movements of the rest of the heavenly bodies are thoroughly known to very few.  And this knowledge, although in itself it involves no superstition, renders very little, indeed almost no assistance, in the interpretation of Holy Scripture, and by engaging the attention unprofitably is a hindrance rather; and as it is closely related to the very pernicious error of the diviners of the fates, it is more convenient and becoming to neglect it.  It involves, moreover, in addition to a description of the present state of things, something like a narrative of the past also; because one may go back from the present position and motion of the stars, and trace by rule their past movements.  It involves also regular anticipations of the future, not in the way of forebodings and omens, but by way of sure calculation; not with the design of drawing any information from them as to our own acts and fates, in the absurd fashion of the genethliaci , but only as to the motions of the heavenly bodies themselves.  For, as the man who computes the moon’s age can tell, when he has found out her age today, what her age was any number of years ago, or what will be her age any number of years hence, in just the same way men who are skilled in such computations are accustomed to answer like questions about every one of the heavenly bodies.  And I have stated what my views are about all this knowledge, so far as regards its utility.