I hold that the first form of a body is the first corporeal mover. But this is light, which as it multiplies itself and expands without the body of m… - Robert Grosseteste

Ostensive demonstration is that which concludes directly to that which is in question. Reduction ad impossibile is that which, when something the opposite of that which is in question has been assumed, concludes with some other proposition directly to a known and manifest impossibility, from the opposite of which the investigator is led back to the original proposition in question. But there is a difference between ostensive demonstration and reduction ad impossibile, because the former proves from things prior in the order of nature but the latter from things posterior in the order of nature. When things prior in nature are better known in the intellect of the person making the demonstration the process is carried out ostensively; but when posterior things are better known to his intellect then the demonstration is carried out per impossibile... in demonstration carried out per impossibile the showing of the original thing in question is carried out by means of things posterior to it in the order of nature... And there is in the contrary, falsely supposed in predicate of subject, a connecting term by which something is implied to be which impossible in the nature of things.

Vacuum stands and remains a mathematical space. A cube placed in a vacuum would not displace anything, as it would displace air or water in a space already containing those fluids.

The diligent investigator of natural phenomena can give the causes of all natural effects... by the rules and roots and foundations given from the power of geometry.