An occurrence is explained by being related to prior events through known laws. Other intellectual activities may also constitute explanation - but this much certainly does. Ideally, an explained occurrence (0) could have been predicted in a connected way - by extrapolation from prior events (e) via the same laws (L). Schematically, 1 Explanation: 0 -Lt, 2, 3-(e e e )'-AI t 2 3 01 Prediction: (e e e )I-L , 2, 3_ +.11 t 2 3 t Thus Mars' backward loop in late summer, 1956, is explained by showing how this follows from (e ) its mean distance from sun and earth, (e ) its t 2 mean period of revolution, (e ) its past positions relative to earth, etc. 3 - by way of the laws of Celestial Mechanics (including (Lt) Kepler's Laws and Galileo's, (L2) Newton's, and (L3) those of Laplace and Lagrange. Moreover, this loop (0) could have been predicted from such events (e -e ) via the laws of Celestial Mechanics. t 3 This is an ideal situation. It crystallized late in the history of planetary theory.The Greeks found explanations for heavenly motions: the back ward loops were explained to their satisfaction. But they could not predict these motions, not in terms of Attic explanatory cosmologies.

Inhalt
Book One - Part I.- Cosmological Explanation, B.C..- The Conceptual Content of Book One, Part I.- The Historical Content of Book One, Part I.- Plato.- Eudoxos and 'Plato's Problem'.- Aristotle.- Book One - Part II.- Ptolemy and Prediction.- Pre-Ptolemaic Anticipations.- Three Dimensional Variations of Ptolemy's Technique.- Book Two - Part I.- The Medieval Rediscovery of Ptolemy's Tool Box.- 'The Ptolemaic System'.- Supplementary Material for Book Two, Section A.- Book Two - Part II.- Copernicus' Systematic Astronomy.- Further Aspects of Copernican Astronomy in Contrast to All that had Gone Before.- Supplement to Section on Copernican Theory.- Book Three - Part I.- Kepler and the 'Clean' Idea.- Supplementary Material for Book Three, Part I.