ARISTOTLE TO AUGUSTINE of change as the passage from potentiality to actuality, whether (as in substantial change) from matter to form or (as in accidental change) from one to another quality of a substance These technical notions, which he employed in such an astonishing variety of contexts, will be examined in detail in later chapters Aristotle’s vision of the cosmos owes much to his Presocratic precursors and to Plato’s Timaeus The earth was in the centre of the universe: around it a succession of concentric crystalline spheres carried the moon, the sun, and the planets in their journeys around the visible sky The heavenly bodies were not compounds of the four terrestrial elements, but were made of a superior Wfth element or quintessence They had souls as well as bodies: living supernatural intellects, guiding their travels through the cosmos These intellects were movers which were themselves in motion, and behind them, Aristotle argued, there must be a source of movement not itself in motion The only way in which an unchanging, eternal mover could cause motion in other beings was by attracting them as an object of love, an attraction which they express by their perfect circular motion It is thus that Dante, in the Wnal lines of his Paradiso, Wnds his own will, like a smoothly rotating wheel, caught up in the love that moves the sun and all the other stars Even the best of Aristotle’s scientiWc work has now only a historical interest The abiding value of treatises such as his Physics is in the philosophical analyses of some of the basic concepts that pervade the physics of diVerent eras, such as space, time, causation, and determinism These are examined in detail in Chapter For Aristotle biology and psychology were parts of natural philosophy no less than physics and chemistry, since they too studied diVerent forms of physis, or nature The biological works we have already looked at; the psychological works will be examined more closely in Chapter The Aristotelian corpus, in addition to the systematic scientiWc treatises, contains a massive collection of occasional jottings on scientiWc topics, the Problems From its structure this appears to be a commonplace book in which Aristotle wrote down provisional answers to questions that were put to him by his students or correspondents Because the questions are grouped rather haphazardly, and often appear several times—and are sometimes given diVerent answers—it seems unlikely that they were generated by Aristotle himself, whether as a single series or over a lifetime 88