Time, said Austerlitz in the observation room in Greenwich, was by far the most artificial of all our inventions, and in being bound to the planet turning on its own axis was no less arbitrary than would be, say, a calculation based on the growth of trees or the duration required for a piece of limestone to disintegrate, quite apart from the fact that the solar day which we take as our guideline does not provide any precise measurement, so that in order to reckon time we have to devise an imaginary, average sun which has an invariable speed of movement and does not incline towards the equator in its orbit. If Newton thought, said Austerlitz, pointing through the window and down to the curve of the water around the Isle of Dogs glistening in the last of the daylight, if Newton really thought that time was a river like the Thames, then where is its source and into what sea does it finally flow?