De cometis

...it is purified and the coarseness is segregated, as in the generations of gems and the operations of alchemy. So that, almost like an elixir, they seem to be absolute from elementary properties. On account of the excellent lordship of the celestial properties from which they deservedly emerge as denotable, they surround these digested and sublimated parts, made suitable for the reception of their virtues and properties, and imbue them with the forces and powers of their own natures. Just as power is provided to the vital spirit and other spirits in the heart. In a similar way, just as generative spirits are produced and born in seeds, and the effective virtues of greater things in themselves. And if those things which are in gems and the properties of the elixir are also left to them, all of which effect their operations through their hidden spirits, so also are comets moved, move, dispose, and alter by their own properties and virtues, suitable for the reception of the properties of their star according to the state existing at the time of generation, not as something future and later. Just as seeds, with the tree rotting or being cut, produce and introduce sperm into the dead, fitting them to themselves.

Having therefore understood the order and the manner in which power is infused into the materials of comets, it behooves one to contemplate further, since gems, herbs, seeds, and other things have hidden virtues not perceptible to the senses, indeed not even comprehensible to our intellect, as Alkindi says. For since all human knowledge arises from the senses, why are the properties of things, which the senses do not perceive, and their productions not a matter of knowledge in reason? These virtues of seeds, the diversities of limbs, organs, complexions, figures, and colors, and of all their qualities, both hidden and manifest, effect their productions according to different species. Thus, gems and herbs similarly, and the virtues of the animal body, through an incomprehensible property, perform wondrous and prodigious effects. Therefore, in all these, philosophers used to say there is a spirit, namely, a subtle substance emerging from a virtuous substance, having the power and force of its virtue wherever it expands, by which, as an instrument, every action of that virtue is accomplished.

This spirit in its acts is assimilated to the sensible species which produce images in the organs proportionate to them through the sensitive power of their objects.

Why, therefore, should the substances of comets and similar things, purified and fortified by celestial virtue, not do the same? Even in the air, diffused in subtlety until they disappear from our eyes, when they effect a spiritual operation just like a seed cut off from a star—to speak metaphorically—having the spirit of virtue; indeed, to call them spirits themselves is possible, though the present discourse does not allow for a dispute on such matters.

ON THE MOTION OF COMETS. 4

then motions are effected

As the material of the comet comes out from the depths of the earth, before it arrives at the flux of the star fixed in the heavens, it is red; often because of the proximity of the smoke and exhalations coming out, like superfluous and segregated matter which remains around the earth, in the blasts of winds, and sometimes by their violent exit, they effect earthquakes. The matter itself, however, participating in the celestial property, by collecting its parts in the air and uniting them, moves by its own power into the place due to it, just as the parts of a bodily foundation and other vital moisture, extended by the property impressed upon them by the power of the member from which they descend, move to a place proportionate to the member's position. And such motion is not to be ascribed to gravity or levity, nor similarly to bodies ascending and descending there. Although the comet also moves according to the motion of the firmament of the day.