Thales

 Thales was one of the first to use observation instead of mythology to define and organize the world around him. He believed that crop failure and floods could be explained by natural events, not angry gods. He used math, not as much as his disciple Pythagoras, to find relationships between things, and theorized that everything that existed came from a single source, which he thought was water since it was required for everything to live. He was wrong of course, but h had the right approach, and at the time it was revolutionary.