Thales the Measurer offers a comprehensive and iconoclastic account of Thales of Miletus, considering the full extent of our evidence to build a new picture of his intellectual interests and activity. Thales is most commonly associated with the claim that ‘everything is water’, but closer examination of the evidence that we have suggests that he could not have said anything of the sort. His real interests, and his real innovations, lay in challenges of quantitative measurement, especially measurements related to the movement of the sun. In this he had no predecessors – and, for centuries to follow, no real successors either. This book is of interest for scholars in the history of philosophy, science, and life sciences. It is aimed especially at researchers in the field, but is also accessible to students and a more general readership.