It is now discovered with certainty, by the researches of later ages, that Venus and Mercury turn round the Sun, and not round the earth, on this subject the ancient system is absolutely exploded. I will now acquaint you with another which provides for every difficulty, one that does not require any amendments of the king of Castile, for its simplicity is so charming that one cannot refuse to believe it. Yours, interrupted the Marchioness, seems a sort of bargaining philosophy; whoever offers a system that is effected at the least expense, has the preference. 'Tis true, said I; we have no other chance of understanding the plan by which the operations of nature are carried on. Nature is a wonderful economist; if a work is to be effected, and two ways are practicable, we may be sure she will adopt that which costs her the least, however trifling the difference. This economy is notwithstanding in every respect consistent with the surprising magnificence which appears in all her productions. Magnificence is employed in the design, and frugality in the execution of it. Nothing should excite our admiration so much as a stupendous project effected by simple means: but we are apt to cherish ideas of a very different kind. We place the frugality in the designs of nature, and her grandeur in the execution. We imagine her forming a contracted plan, and executing it with ten times the labour that is requisite: what can be so ridiculous? I hope, she replied, that the system you are going to explain will strictly imitate nature; the simplicity you so admire will spare me a great deal of trouble in comprehending your instructions. Your hope will be realized, said I, we have now no useless incumbrances. At the appearance of a certain German named Copernicus,[9] astronomy became simplified; he destroyed all the unnecessary circles, and crushed to pieces the crystalline firmaments.[10] Animated with philosophic enthusiasm, he dislodged the earth from the central situation which had been assigned it, and in its room placed the sun, who was more worthy of such a mark of distinction. The planets were no longer supposed to perform their revolutions round the earth, and enclose it in the centre of their orbits. If they afford us light it is as it were by chance, and in consequence of passing us in their course. They all turn around the sun; the earth itself not excepted; and as a punishment for the indolent repose it had been thought to enjoy, Copernicus made it take an ample share of the general activity: in short of all these celestial attendants, appointed for the service of our little globe, the moon alone is left to move round it. Stop a moment, said the Marchioness, your imagination is so elevated with your subject, you have explained it in such pompous language, that I believe I have scarcely understood you. The sun, you say, is immoveable in the centre of the universe; which of the planets is next in succession? 'Tis Mercury, I replied.