7 GALILEO, HUYGENS AND THE PENDULUM CLOCK: ISOCHRONISM AND SYNCHRONICITY 1 INTRODUCTION Filip BUYSE  On the 2 nd October 2017, the Noble Prize in Physiology or Medicine was awarded to three researchers who were able to elucidate how the internal, biological clock of living organisms adapts itself so that it is synchronized with the Earth’s revolutions. In the press release of the Noble Prize committee we read the following introductory paragraph: Life on Earth is adapted to the rotation of our planet. For many years we have known that living organisms, including humans, have an internal, biological clock that helps them anticipate and adapt to the regular rhythm of the day. But how does this clock actually work? Jeffrey C. Hall, Michael Rosbash and Michael W. Young were able to peek inside our biological clock and elucidate its inner workings. Their discoveries explain how plants, animals and humans adapt their biological rhythm so that it is synchronized with the Earth’s revolutions 2 . Christiaan Huygens (1629-1695) was the first physicist to observe and analyze the phenomenon of synchronization. More precisely, the Dutch physicist and astronomer observed on the 1 st March of 1665 3 that two pendulum clocks which were standing in front of him started to move in phase. He couldn’t believe his eyes and tried to find a mechanical explanation for this spectacular observation “which no one ever would have thought of. “. Initially, he interpreted ‘l’accord merveilleux’ as a kind of ‘sympathy’ but already one month later 4 he discovered the real mechanical cause of this odd phenomenon. In this volume, Dr. Kurt Wiesenfeld explains how his research group has examined synchronization by means of reconstructions of Huygens’ pendulum clocks. In another paper, Dr. Filip Buyse argues that Spinoza was in contact with Christiaan Huygens during the period of his spectacular invention. Hence, the Dutch physicist and astronomer might have influenced and inspired Spinoza (1632 -1677) in his views on the agreement between bodies in the universe. This would resolve Spinoza’s otherwise paradoxical phrases in his answer to Robert Boyle’s question, in his Letter 32 (1665) to the secretary of the Royale Society. Furthermore, Dr. Maxime Rovere  Visiting Fellow at the Centre for the History and Philosophy of Physics at St Cross College, University of Oxford, 61 St Giles’, Oxford OX1 3LZ, UK. Visiting Researcher at the Vossius Centre of the University of Amsterdam. E-mail: f.a.a.buyse@gmail.com