She smiled somewhat, and then she said, “If the opinion of mortals errs where the key of sense unlocks not, surely the shafts of wonder ought not now to pierce thee, since thou seest that the reason following the senses has short wings. But tell me what thou thyself thinkest of it.” And I, “That which here above appears to us diverse, I believe is caused by rare and dense bodies.” And she, “Surely enough thou shalt see that thy belief is submerged in error, if then listenest well to the argument that I shall make against it. The eighth sphere[1] displays to you many lights, which may be noted of different aspects in quality and quantity. If rare and dense effected all this,[2] one single virtue, more or less or equally distributed, would be in all. Different virtues must needs be fruits of formal principles;[3] and by thy reckoning, these, all but one, would be destroyed. Further, if rarity were the cause of that darkness of which you ask, either this planet would be thus deficient of its matter through and through, or else as a body distributes the fat and the loan, so this would interchange the leaves in its volume. If the first were the case, it would be manifest in the eclipses of the sun, by the shining through of the light, as when it is poured out upon any other rare body. This is not so; therefore we must look at the other, and if it happen that I quash this other, thy opinion will be falsified. If it be that this rare passes not through,[4] there needs must be a limit, beyond which its contrary allows it not to pass further; and thence the ray from another body is poured back, just as color returns through a glass which hides lead behind itself. Now thou wilt say that the ray shows itself dimmer there than in the other parts, by being there reflected from further back. From this objection experiment, which is wont to be the fountain to the streams of your arts, may deliver thee, if ever thou try it. Thou shalt take three mirrors, and set two of them at an equal distance from thee, and let the other, further removed, meet thine eyes between the first two. Turning toward them, cause a light to be placed behind thy back, which may illumine the three mirrors, and return to thee thrown back front all. Although the more distant image reach thee not so great in quantity, thou wilt then see how it cannot but be of equal brightness. [1] The heaven of the fixed stars. [2] If all this difference were caused merely by difference in rarity and density. [3] The stars exert various influences; hence their differences, from which the variety of their influence proceeds, must be caused by different formal principles or intrinsic causes. [4] Extends not through the whole substance of the moon. “Now, as beneath the blows of the warm rays that which lies under the snow remains bare both of the former color[1] and the cold, thee, thus remaining in thy intellect, will I inform with light so living that it shall tremble in its aspect to thee.[2]