De cometis

...to those not knowing mathematics. To those who do know, none of these things are doubted. Let us not wonder, therefore, if the substance of such quantity is the cause and sign of the greatest events, from which not only it itself, but also much more, the superfluitites separated from it, as appeared in the first part, flew away with it, irrigating the air. How, therefore, will anyone doubt concerning the greatest changes and wondrous events, who view the air's support?

On the cognition of the comet from the posterior, from motion, figure, and color

The proper motion of the comet was no more similar to the motion of any star than to the motion of Venus at the time of its appearance, since it moved itself into the north, and retrograde. Such also, as was apparent, was the motion of the comet. For none of the others moved into the north, except Jupiter, but he was direct at the beginning of the appearance. This cognition, as appeared from the first part, is the most powerful a posteriori from effect to cause. Then the figure of this comet was similar to that which the wise attributed to comets of the nature of Venus, since it had a long ray and hair similarly diffused all around. The color, although to some it appeared black, and to others blue, as they assert in their writings; to others, however, whose sharpness of sight I thought to be trusted, it seemed white to me as well. And because we could be deceived in vision concerning color, it does not, however, vary its species and nature, just as motion and figure do not. For nature is the principle of motion, through which, as is said in the third book of Physics Aristotle's Physics, nature ought to be known. By these, a posteriori, the comet is judged to be of the nature of Venus. And it is called a cloud.

On the cognition of the comet a priori from cause to effect from the configuration of the stars

A priori, however, it was known by this way: because, as has been grasped, the applications of great signification were before the appearance of the comet, such as one in August, in which Venus was stronger and more potent than all other planets. Then at the end of the following November, there was an eclipse of the moon in Gemini, in which victory was afforded to Mars over Saturn, whose disposition it was dissolving, wherefore it was dilating the pores of the earth and breaking up the constriction made by Saturn. Whence, in the days immediately following, an impetuous multitude of winds exited from the earth, whose matter had been segregated in the preceding times in the generation of the comet as gross and superfluous. After whose exit, the comet immediately appeared. The substance of the comet, therefore, was generated before these winds, and a substance of such quantity did not require a little time for its digestion through some strong constellation preceding it. But no greater one preceded than that of Saturn and Mars at the same time with the conjunction of Jupiter and Venus, in which, as was said, Venus was stronger than all. Its nature, therefore, was disposing the elements and the elemental compounds, and each of the mixtures which were beginning then. And at that time, the substance of the comet began to be generated. Wherefore, I conclude that it holds the nature of Venus, aided by being under Libra, where it first appeared, which is the house of Venus and the place of the conjunction of Venus and Jupiter, before other parts of the sky.