The history of thought knows many barren truths and fertile errors. Kepler’s error turned out to be of immense fertility. ‘The direction of my whole life, of my studies and works, has been determined by this one little book,’ he wrote a quarter-century later.16 ‘For nearly all the books on astronomy which I have published since then were related to one or the other of the main chapters in this little book and are more thorough expositions or completions of it.’
