Three events, which happened all within the same week some ten years ago, set me on the track which the book describes. The first was a reading of Emile Meyerson works in the course of a prolonged research on Einstein's relativity theory, which sent me back to Meyerson's Ident­ ity and Reality, where I read and reread the striking chapter on "Ir­ rationality". In my earlier researches into the origins of French Conven­ tionalism I came to know similar views, all apparently deriving from Emile Boutroux's doctoral thesis of 1874 De fa contingence des lois de la nature and his notes of the 1892-3 course he taught at the Sorbonne De ['idee de fa loi naturelle dans la science et la philosophie contempo­ raines. But never before was the full effect of the argument so suddenly clear as when I read Meyerson. On the same week I read, by sheer accident, Ernest Moody's two­ parts paper in the JHIof 1951, "Galileo and Avempace". Put near Meyerson's thesis, what Moody argued was a striking confirmation: it was the sheer irrationality of the Platonic tradition, leading from A vem­ pace to Galileo, which was the working conceptual force behind the notion of a non-appearing nature, active all the time but always sub­ merged, as it is embodied in the concept of void and motion in it.

I: The Tradition.- One: Aristotelian and Platonic Conceptions of Explanation.- Two: Aristotle's Philosophy of Nature and Theory of Potentiality.- Three: Plato's Concept of the Actual and His Philosophy of Nature.- II: The Logical Revolution.- Four: The Copernican Harmony.- Five: Bacon's Informative Logic.- Six: Informativity and Paradox: Galileo's Conception of the Nature of Physical Reality.- Seven: Descartes' Informative Logic.- III: Newton's Physics and its Critics.- Eight: Actual Infinity and Newton's Calculus.- Nine: Newton's Logic of Space and Time.- Ten: Modern Newtonian Historiography and the Puzzle of Newton's Absolute Space.- Eleven: Absolute Motion and the Nature of Inertial Forces.- Twelve: Locke and the Meaning of "Empiricism".- Thirteen: Newton's Invention of the Problem of Induction.- Fourteen: Circularity and Newton's Philosophy of Nature.- Fifteen: Leibniz's Aristotelian Philosophy of Nature.- Sixteen: Berkeley's Aristotelian Critique of Newton's Physics.- Epilogue.- Appendix: Some Basic Ideas in Newton's Physics.- Notes.