De cometis

it has two rays, so however that it appears like a column. Similarly, it is not determined but is for the most part ascribed to the dragon's head. It is not very bright. Co. of a leaden or ash-gray color, having two rays sometimes behind it, quite long, and sometimes two before it, not long. And it is thick in the middle, and some have ascribed it to Jupiter, and it appeared more rarely than the others.

On the knowledge of comets a posteriori and "that it is"

Our knowledge depends on things sensed, wherefore it behooves us to know such things and celestial virtues in all these mentioned things from accidents, which contribute a great part to knowing what it is, as Aristotle says. For their motion and those things which are seen to follow any species, by the considerations of the philosophers, led them to the reasoning of them and the knowledge, namely, when some comet was seen to be moved with a motion similar to the motion of Mars, and they were seen also to follow deeds similar to the properties of Mars, they said it was of the nature of Mars. Such comets, also, appearing many times under a similar figure in a similar motion with similar events following, they ascribed to this species of comet. Therefore, the most potent sign in presenting knowledge is motion, the way by which we are able to know, since nature is the cause and principle of motion, as is clear in the second book of the Physics. Certainly, therefore, we shall know more effectively and perfectly from motion, Aristotle saying in the third book of the Physics that, the motion being ignored, it is necessary to ignore the nature. Then from the figures which the wise have ascribed to the species of comets, then and finally from color, which is less certain. But of all, the most certain sign is from the parts of apparent effects and accidents contemplated and related to the properties of the planets. After these effects, the wise distinguished the name and species of the comets mentioned before. This way of knowing, however, does not suffice here, since it is necessary, before the arrival of such events, to determine its nature and those things which we conjecture will be future from it.

On the knowledge a priori and "on account of which"

It is also necessary to consider the way of knowing "on account of which" a priori from causes, which order of knowledge would be most potent. Therefore, one must consider Mars before the arrival of the comet, at what time it applied to Saturn, and if it fortified the moon in an airy place; without this, as has been said, it does not appear. And which of the planets was then stronger in its place and its figure to others before such an application; for such a planet has great power in the preparation of the nature of the comet and in the infusion of its virtues, and then the others which are said to mix with it. Haly says, upon the second Quadripartitum a treatise on astrology by Ptolemy, that the quality of the approach is had from the form and color of the comet and its convenience to the planets. For it is its accident from the nature of that planet with which it has more agreed, and according to the virtue of the one who governs. And from the mixture of the planets among themselves, by conjunction or opposition, or another principle preceding, the comet approaches. It will be necessary, therefore, to mix these knowledges, since if the planet governing before the arrival is that to whose motion the comet seems to be moved, the figure also not being much diverse from that which the wise ascribed to it, and the color also being proportionate, it is most certain, nor is it necessary to seek another; for it is produced by the virtue of the comet. And for a more notable application of such a planet at the time of its strength before the arrival of the comet, all accidents are regulated and determined according to the application of other planets to it, and according to the figure of the sky in diverse regions.