Full Idea
Thales was the first thinker in the west to believe that the arché (the basis of things) was intelligible.
Clarification
The 'arché' is the source or basis of everything
Gist of Idea
Thales was the first western thinker to believe the arché was intelligible
Source
comment on Thales (fragments/reports [c.585 BCE]) by David Roochnik - The Tragedy of Reason p.138
Book Reference
Roochnik,David: 'The Tragedy of Reason: the Platonic logos' [Routledge 1990], p.138