Thales
According to Bertrand Russell, "Western philosophy begins with Thales".  Thales attempted to explain natural phenomena without reference to mythology and was tremendously influential in this respect. Almost all of the other pre-Socratic philosophers follow him in attempting to provide an explanation of ultimate substance, change, and the existence of the world—without reference to mythology. Those philosophers were also influential, and eventually Thales' rejection of mythological explanations became an essential idea for science.  He was also the first to define general principles and set forth hypotheses. As a result he has been called the "Father of Science", though maybe Democritus is more deserving of this title.