>I can't see how eating spinach or not has any real effect on society. Some values are more important than others, and some are even more important than facts. When you said, 'Doesn't the "rigorous approach to seeking/knowing truth" seem kind of detached from the reality of life to you?', you seem to be placing values over facts. That doesn't make values factual though.

Sure, my point wasn't that the yuckiness of spinach and the purpose of life were equally important issues, but that on the view that oughts are simply matters of opinion, that my rejections of the claims "spinach is yucky" and "our purpose is to reproduce" are equally valid---I've simply disagreed with an opinion of yours. This is very different from me rejecting a claim like "The earth is round". And I think, with claims like the latter, it makes sense for a rational discussion to be had about whether or not such a claim is right. But with claims like "spinach is yucky", there's surely nothing to be discussed---I like spinach and you don't, end of story. But I think you sense that the purpose of life isn't this kind of claim; otherwise it wouldn't make sense to try to rationally discuss this claim.

So I suppose I'm struggling a bit to understand your case---you think the question of purpose is just a matter of opinion, but at the same time, our purpose is clearly to reproduce? Is this not just your personal opinion, on your view? Perhaps you're saying that, as a matter of fact, that everyone will find fulfillment in reproducing? But even this isn't true, there are plenty of celibates who find fulfillment in their way of life. Maybe just that most people will find fulfillment in reproducing? Maybe so, though there's clearly a lot of things other than reproducing that people find fulfilling---science, religion, mathematics, art, music, sports, and so on. Maybe that an unconscious reproductive drive undergirds the desire to engage in such activities even if they are not immediately related to reproduction directly? OK - maybe so, but if all of human behavior is undergirded by a reproductive drive, "our clear purpose is to reproduce" doesn't actually tell us anything about what to do, since anything that we do would be distantly related to a reproductive drive, and we're back to square one.

(Of course, that's all putting aside that that all says nothing about what our true, objective purpose really is.)

>They're wrong from a modern standpoint. The problem is that morals are like fashions; they are subject to change, and by that I mean, the weight that we place on different values is always changing.
>They appear wrong from our current standpoint.

I don't see how the fact that beliefs about morals have varied across time/space/culture means morality must be subjective any more than the fact that beliefs about scientifically discoverable facts have varied across time/space/culture means that scientifically discoverable facts must be subjective. It doesn't matter that there are some people who defend the earth being flat today. It doesn't matter that people used to believe the sun revolved around the earth. These things don't make beliefs about the shape of the earth or the relationship between the earth and the sun fashions in the sense that there's no truth of the matter. So too for morality.

>You cannot scientifically demonstrate that slavery is wrong though.

Maybe so, but I don't think scientific demonstrability is a necessary condition for something to be objectively true.