"And "we make our own purposes" may sound nice, but it's pretty clearly not an answer to the question.
Survival would, from a Darwinian perspective, appear to be a 'legitimate' purpose. All other purposes, from that perspective, would have to be artificial constructs. It would seem to me, then, that there could be no objective way of evaluating their relative merits beyond their respective capacities to amuse me."
'Survival' is just an urge we have inherited from our Darwinian origins, if you want to look at it that way. But why should we follow the dictates of some inefficient natural system anymore than we should insist that we cannot use technology since it was artificially created?
Artificial just means "man made" not "fake."
As I see it, pursuit of a greater human society in conjunction with pursuit of the greatness of the individual is the goal of morality. And so much of what makes us great - or can make us great - are things artificially created. In fact, the things that make me most proud of being human are things that human beings have made and done and not the powers we have inherited from nature.
"Rather high-falutin' stuff, Orthoprax. But I have some difficulty distinguishing it from anybody else's high-falutin' stuff.
In the end, Plato, Gandhi, Stalin, Hitler, Jefferson and a host of others all sought a "greater human society." To some extent or another, each relied on exalting one or more individuals to attain his end. I need more than that kind of phrase to evaluate whether a society is great, or a philosophy is any good."
Indeed, but where they failed was in understanding the basic equality and fairness that must logically be employed for essentially identical objects. All people are essentially the same in this moral sense and therefore ought to be treated similarly.
It is only when you are operating under bias or incorrect data that you can conclude that some minority should be treated fundamentally differently - and therefore unfairly - from the majority.
This, of course, though is only a general principle and there are exceptions - like age or mental handicaps and so on where the minority is truly different and therefore deserves distinct standards.
The point is, and I think I made this clear the first time, that the greater human society is not a moral goal by itself but only makes sense with the complementary goal of the greatness for the individuals who compose society on their own terms. There is a symbiotic relationship between the individual and society and pursuing one at the expense of the other ultimately damages both.