I want to point out what may be two incorrect presuppositions in the question. First:
I am referring [...] to [...] the [philosopher] who deserves the most credit for advancing human philosophy towards being more true.
There may be an assumption here that the work of "advancing philosophy towards being more true" is mostly done by philosophers (in which case, it's reasonable to ask which of those philosophers has done the biggest slice of that work). But actually it seems likely, at least to me, that if you look at present-day philosophical opinions and long-ago philosophical opinions, pick out the changes about which we can be most confident that today's opinions are better, and ask how it came about that philosophers used mostly to believe X but now mostly believe Y -- then in many cases the reasons would be largely down to progress in (what is now called) science rather than in philosophy.
So if the question were altered slightly, asking for the person rather than specifically the philosopher who deserves most credit for advancing human philosophy towards being more true, people like Eratosthenes and Darwin and Einstein might be better candidates than the likes of Socrates and Hume and Russell.
The second possible incorrect presupposition is that "being more true" is the best way to compare the merits of philosophical positions. Isn't it possible, and even likely, that many philosophical questions are ones for which there is, actually, no fact of the matter about whether one answer or another is correct? Some philosophical questions might be important even if they don't have true answers as such; answering them one way rather than another might lead to greater happiness or scientific progress or something.
In such cases it might actually be best to find a way to reframe the debate so that it doesn't take the form of arguing about truth and falsehood. (So, for instance, suppose it turns out that moral realism is wrong, so that many, many philosophical debates about substantial questions of ethics were asking questions that have no true answers; it might then be better to ask questions like "what moral principles tend to lead to stable societies and happy lives?" or something.)
In terms of what counts as "philosophy", I was using a fairly modern conception of it. If that is granted (although come to think of it I could be challenged on that), then it follows that modern scientists have generally not considered such questions- Einstein, Darwin etc have not considered questions that fit into the modern idea of philosophy.
I also see no reasonable way to argue that there is "no fact of the matter" about whether morality is true or not. The basic idea of "just plain right/wrong" is incoherent and therefore clearly wrong in the same sense free will is clearly wrong.
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?