Is it possible that a person of modest intelligence could learn the whole history of philosophy, in terms of knowing every notable philosopher (from Thales to, say, Rorty), having read a few of their books or at least knowing and being able to expand upon their positions ... or is it simply outside the scope of a person, any less than a genius to have the time to gain such knowledge? It seems to me that there is not more than a couple of hundred such philosophers, and as such could be accomplished, at least superficially. Or is it more efficient to decide outright to miss some philosophers out?
Great question! By the way you pose the question (Thales to Rorty) I assume you mean western philosophy. Yes, I think you can carry out such a project, reading a bit of each of the major philosophers and then relying on a good history as a guide. I would highly recommend Anthony Kenny's multi-volume Oxford University Press books as lively and engaging. Copleston's history of philosophy is perhaps less engaging but it is reliable and a good companion. Speaking of Companions, Blackwell, Oxford, Cambridge, and Routledge each have massive Companion series that would also be helpful in filling out your reading. You might want to set as a goal an overall grasp of the history of philosophy and then dig in to a few areas and thinkers so as to deepen your understanding of philosophy and also to engage more in the practice of philosophy (wrestling with arguments and counter-arguments) in reference to a specific area or philosopher.