He first explains the three methods of heat generation:
- An object that is hot
- The scattering of rays
He decided that Method 2 is also insufficient to explain the heat, because the motion that creates heat is caused by two substances moving in opposite directions—for instance, rubbing your hands together to warm them up—and the sun's circular motion does not act upon a second substance moving in an opposite direction: everything up there moves from east to west.
Method 3, he decides, must pertain. He reminds his reader that Euclid explains how a concave mirror can focus the sun's rays to cause a fire. He states that the sun's rays falling upon the earth are scattered, but reflection by a mirror or refraction by a (clear) spherical body can change the direction of the rays, focusing them via the medium of the dense air and generating heat. For him, this has much to do with the denseness of the medium: he tells us that the same amount of light falls on a mountaintop and scattering can be observed there, but the thinness of the medium of air disallows the generation of heat.