Today is one of those days when I write something more intelligent on someone else's blog than I do on my own, so today, I'm going to copy and paste a comment I left over at Malcolm's fine blog. Read Malcolm's post first for background before you read what I've pasted below. (NB: The text below was ever-so-slightly edited for style.)
[Malcolm, you] might be very interested in philosopher Nicholas Rescher and his notion of "orientational pluralism," which is what your post seems to be hinting at or leaning toward. I've blogged about Rescher in the context of religious pluralism; his ideas were appropriated by conservative religious thinker S. Mark Heim, who wanted to offer a conservative answer to religiously liberal thinker John Hick's classic notion of "convergent pluralism" (i.e., all religious traditions are equally legitimate paths up the mountain to the same summit/fulfillment). Heim's "divergent pluralism" is a deeper pluralism that doesn't converge on any transcendent One, but Heim also affirms that it's rational for anyone from a given perspective to view the world through the filter of his religious or philosophical position.
One theme I harped on, back when I wrote often about religion and religious issues, was the idea that core religious notions and religious truth-claims are hegemonic in nature: what is declared or professed is automatically meant to apply to all. Christ didn't die only for Christians' sins: he died for everyone. Buddhist sunyata (emptiness) isn't a metaphysical truth only for Buddhists: it's an objective truth about the nature of reality. Shiva doesn't exist merely for Saivites: he exists for, and is knowable by, us all.
Heim's pluralism relies on an analogy that's similar to your fish/cat image above: he uses a "travel" analogy. Imagine that you're in DC and you have to go to Alexandria, VA (my hometown), only 14 miles away. What's the best, most expedient, most sensible/rational way to get there? Now imagine you're in Bogota, Colombia, but you have to go to Oahu, Hawaii. The two paths are nothing alike in terms of their start and end points; they're also nothing alike in terms of the means we use to reach each respective destination. True: these are both forms of travel, but to say that these two paths are therefore exactly alike is to ignore the fact that "mere details" cannot be swept aside in our rush to embrace abstract similarities: the details are, in fact, constitutive of each path and cannot be glossed over as if they were beneath consideration.
Being enveloped in water makes sense for the fish; being surrounded by air makes sense for the cat. Taking a plane to Hawaii makes sense if you're in Bogota; driving to Alexandria makes sense if you're in DC. This is a sort of natural incommensurability (you can't expect the cat to live and flourish under water) that Heim/Rescher's pluralistic paradigm considers, but Hick's convergent paradigm does not.
I'm not saying I totally agree with either Heim or Hick, but there we are. Worldviews are rationally incommensurate and hegemonic by nature.
All this has implications for Dr. Vallicella's claims. We can speak of universal values because, from the perspective of anyone with any values at all, that person's values are seen as universal. But this fails to solve the objective problem of whether one overriding set of values actually takes precedence over all the rest. Hick would say that one set does, in fact, take precedence, and it's the set toward which we're all converging (in his own way, Steven Pinker seems to acknowledge the possibility of a Platonic moral realism); Heim would say that it's rational for everyone to think that his own value set is the one that takes precedence, and the best we can do is to acknowledge that other axiological perspectives are both possible and rational as well.