I was almost driven to madness in considering and calculating this matter. I could not find out why the planet would rather go on an elliptical orbit… - Johannes Kepler

Nothing which consists of corporeal matter is absolutely light, but that is comparatively lighter which is rarer, either by its own nature, or by accidental heat. And it is not to be thought that light bodies are escaping to the surface of the universe while they are carried upwards, or that they are not attracted by the earth. They are attracted, but in a less degree, and so are driven outwards by the heavy bodies; which being done, they stop, and are kept by the earth in their own place.

If the attractive virtue of the moon extends as far as the earth, it follows with greater reason that the attractive virtue of the earth extends as far as the moon and much farther; and, in short, nothing which consists of earthly substance anyhow constituted although thrown up to any height, can ever escape the powerful operation of this attractive virtue.

Nu ne întrebăm în ce scop util cântă păsările, cântecul este bucuria lor de când au fost zămislite pentru cântat. La fel, n-ar trebui să ne întrebăm de ce spiritul omului se străduieşte să pătrundă secretele cerurilor. Diversitatea fenomenelor naturii e atât de mare, iar comorile ascunse în ceruri atât de bogate, tocmai pentru ca spiritul omului să nu fie nicicând lipsit de hrană proaspătă.