Thales of Miletus (the so-called 'Father of Science') seems to be the first philosopher of the Greek tradition.
Now, obviously, he was wrong about nearly everything, but the leap from being silent to being wrong may have been humanity's greatest step towards being more true.
I'd (idiosyncratically) consider Thales' pupil Anaximander the first philosopher, as we understand 'philosopher' today, and consider Thales more the ur-scientist. From what little information remains about them, Anaximander's doctrines were more traditionally metaphysical than Thales', and he was a deeper and more systematic thinker; we also have a single line of actual prose from him, the oldest extant piece of philosophy in human history (excepting maybe some stuff from the Upanishads).
Since LessWrong is a major congregation point for certain philosophical ideas, and because people here tend to be more objective (in the sense of not being self-deluded) than elsewhere, I thought I'd ask people's views.
To be clear, by "Greatest Philosopher" I am referring not to the most correct philosopher in human history but the one who deserves the most credit for advancing human philosophy towards being more true.
Off the top of my head I would say that a prime candidate would be Hume- amongst other things he rejected the idea of a soul, realised to a much greater extent than his predecessors the limits of human knowledge, and opposed the idea that reason is somehow an objective force that can make priorities independent of emotions.
Aristotle deserves considerable credit relative for his time but doesn't make the list because although it wasn't his fault his ideas were dogmatically accepted and held back both science and philosophy later on.
Your thoughts?