In both devotional matters and mundane ones, the same principle applies. When a person sees a man entering the mosque shortly before the Friday Prayer, clad in the garments ordinarily worn for worship, he concludes that the man has come to pray, for that is what people of this category customarily do. Likewise, if he sees someone seated in the place reserved for ablution and beginning the acts that those purifying themselves perform, he recognizes that the man is performing ablution.
Customs, however, are sometimes broken. One may also learn what an agent is doing from knowledge of his attributes, or infer his attributes from his actions. Yet that belongs to the realm of rational, speculative argument—deducing the consequent from its inseparable concomitant—and is not part of the intuitive experiential knowledge we have been discussing.
Philosophical Sciences as Experientials
Most of what the philosophers—and, indeed, rational people in general—hold by way of universal knowledge concerning existing things comes from observing the habitual behavior of those things. This falls under what they themselves call “intuitive propositions.” The bulk of their intellectual disciplines in natural philosophy and astronomy, such as the science of celestial configuration (ʿilm al-hayʾa), likewise belongs to this experiential class, if it deserves to be called knowledge at all. For in these subjects they possess no demonstrative proof that can compel an opponent; they have no conclusive argument against one who disputes them.
Furthermore, the claim that these natural bodies have been subjected to experiment, or that celestial motions have been observed experimentally, is known to most people only through hearsay, and rarely through mass-transmitted reports. At best, the alleged experiments are passed down from a handful of physicians or calculators.
The science of celestial configuration, the greatest of their mathematical disciplines—whose practitioners Aristotle regarded as the leading philosophers and whose doctrines he considered the very core of philosophy—derives from it our knowledge of the number of the celestial spheres and their motions. Sense perception, by contrast, reveals only the motion of particular bodies: we see the sun moving, the moon moving, and the planets moving—nothing more. As for the claim that these bodies are carried in revolving spheres, that can be known only through this science.
Their ranking of the spheres one above another was inferred from eclipses in which the higher body covers the lower. Thus, when they observed the moon eclipsing all the others, they concluded that it lies beneath them all. Conversely, when they saw that Saturn never eclipses anything while everything else can eclipse it, they inferred that Saturn is situated above the rest.