  Knowledge of what is “for the most part” includes knowledge concerning eclipses, and eclipses, of course, only rarely occur. How can there be knowledge of, and necessary truths about, what, most of the time, does not even exist? For one thing, eclipses are always present in their “causal reasons,” that is, in the motions of the sun and moon, which guarantee that eclipses occur now and again, and, moreover, we can say that it is always true that if a certain situation occurs, in which no impediments are present, an eclipse will occur necessarily due to the nature of the Moon. (Grosseteste (1981) I 18, lines 189-214.)