When Aristotle opened his eyes and looked up into the night sky, much like you or I might look up into the night sky, he seems to have believed he was looking at intellectual perfection. The moon, sun, the wandering planets, and the fixed stars all appeared to travel with perfectly circular, uniform velocity. Or, at least, Aristotle thought the celestial objects should travel with perfectly circular, uniform velocity. (The circle is the only perfect shape, since it is made of a single continuous line.) Slight deviations from the expected norm could be overlooked in the wake of overwhelming weight of evidence. The sun rose and set, and rose again. The moon traveled similar paths across the night sky. As did the wandering planets, though at much slower paces. And the fixed stars returned to their original configuration once a year.
By contrast, terrestrial motion was decidedly finite, linear, and imperfect. Aristotle's use of the term motion encompasses far more than that for which we use the term. Aristotle's definition of the term would include the growth of a plant in addition to a change of spatial location. Terrestrial objects, he believed, all tended towards their natural place of rest. Heavy object fell downwards; airy objects rose upwards.
This rough outline of Aristotelian basics should be recognizable to each of us. Even if we don't actually think like Aristotle, we understand how he drew his conclusions.
Not so recognizable, I suspect, understanding of human nature that goes along with this account of the physical universe. At the beginning of his Metaphysics, Aristotle says the human being delights in their senses, especially the sense of sight. The bodily senses (seeing, hearing, touching, smelling, and tasting) yield bits and bytes of information for the mind to think upon. Without the mind's discriminatory powers, separating truth from falsehood, actuality from mere appearance, however, the bodily senses are powerless to yield any sort of complex judgment about the way things are.
Which is to say that perfection attaches itself to the operations of the mind and imperfection to the bodily senses. When Aristotle looked around at terrestrial objects, he thought that both the bodily senses and the terrestrial objects of the bodily senses possessed a certain kind of imperfection, which was ultimately impenetrable to human reason. Material things came into being and passed away. They were therefore, in an ultimate sense, inconsequential. On the other hand, what might be gleaned from individual things about general features shared in common by whole classes of individual things was consequential. For these reflected the unchanging nature of perfect truth, rather than the changing nature of individual things. And when Aristotle looked up into the night sky, he saw an image of the mental perfections that he found in human rationality. As he says in On the Heavens,
Our theory seems to
confirm experience and to be confirmed by it. For all men have some conception
of the nature of the gods, and all who believe in the existence of gods at all,
whether barbarian or Greek, agree in allotting the highest place to the deity,
surely because they suppose that immortal is linked with immortal and regard
any other supposition as inconceivable...There mere evidence of the senses is enough to convince us of
this, at least with human certainty. For in the whole range of time past, so
far as our inherited records reach, no change appears to have taken place
either in the whole scheme of the outermost heaven in in any of its proper
parts…The same ideas, one must believe, recur in men’s mind not once or twice
but again and again. (402-3; I.3[270b])