Monism Vs. Pluralism
Pluralism has a certain prima facie plausibility -- it does seem like there are lots of different things we care about. But does that intuition represent a fundamental moral truth, or is it just the product of being socialized in a culture that does not have one clear and consistent moral philosophy? A sense of the answer, I think, has to come from asking what we do when two of our ostensibly different values conflict and we're faced with a choice between them.
The most obvious response is to set up some sort of tradeoff. We can say, for example, that we're willing to accept a loss of X amount of beauty if it gains us Y amount of happiness. But as soon as such a tradeoff rule is established, we're on the express train to monism. If two values trade off at a defined rate, then we can express this in terms of them being measurable in a common underlying quantity. And making the tradeoffs vague, denying the ability to precisely define either the tradeoff ratio or the values of particular choices, simply weakens the ethical system's ability to give clear answers without eliminating its monistic implications. Granted, there's still a distinction to be made between substantive monism and abstract monism. Substantive monism, like classical utilitarianism, reduces all values to some real phenomenon -- such as the psychological state of happiness -- by examining the causal connections between other things we value and the phenomenon in question (e.g. the value of freedom is determined by the amount of happiness it leads to). Abstract monism posits some conceptual metric, perhaps just called "value," in which the relative worth of different substantive things can be expressed. Abstract monism need not even posit the ontological reality of this common metric -- "value" can be a pragmatist device for moral calculation, rather than a Platonically real Idea. But it is still effectively monism. Tradeoff rules, while immensely useful in guiding actual action, deprive us of the high-minded refusal to make comparisons or sacrifice one thing for another that is at the root of much of pluralism's appeal.
Another option is lexical ordering. We can rank the values, and then in any situation the first value must be equal before we consider the second. For example, if freedom is our primary value, then we always pick the option that maximizes freedom. But if two or more options are equal in terms of freedom, then we ask how they stack up on our second value (perhaps beauty). And if there are still two options equal in both freedom and beauty, we might move down the list to happiness. Etc. This works best if our values are yes/no characteristics (e.g. "are anyone's rights violated?") or satisficing ("ensure everyone has Z utils of happiness -- extra happiness above Z is not morally of concern"), rather than maximizing continuous variables. Otherwise it's highly unlikely you'll ever get to use even the #2 value -- making your system monism in practice even if conceptually there are other values out there. And even if your values are satisficing, the chance of a value ever having an impact on your choices quickly becomes vanishingly small as you move down the list. So while lexical ordering technically avoids tradeoffs, it goes in the opposite direction from where the pluralist intuition guides us.
So let's say we have multiple values, which are incommensurable (can't be traded off) and equal (can't be ranked). One common claim is that when faced with a value conflict, rather than following an ethical rule (which is usually implied to be a slavish, mechanical form of action), we should make a choice that expresses our character-- "look at me, I'm the kind of person who will sacrifice happiness for freedom." This strategy maintains pluralism at the global level by sanctioning relativism at the individual level. That is, each person's character drives them to be a monist, or at least a lexical orderer, even though different people are different sorts of monists. This raises the question of on what basis a person chooses what kind of value-promoting character to express. Assuming it's not chosen entirely at random, nor is it a not-open-for-debate product of their genetic or socialized temperament (talk about slavish and mechanical action!), a person presumably has reasons for their choice -- reasons that could be used to judge others as having bad character and/or to exhort them to express the proper character. But if that's the case, we're back in full-on monist territory (albeit with a two-level structure that allows us to signal some uncertainty about the precise form of the monism while engaging in meta-discussion).
Finally, we could deny that ought implies can. This discussion has been assuming all along that in any situation, at least one option is morally justified. We can always discern a lesser evil -- or else we're justified in picking a greater evil because no lesser evil is available. But perhaps pluralism means that there are situations in which every option is wrong, in which you're damned if you do and damned if you don't. An analogy to physical incommensurability makes me think this is the most internally consistent form of pluralism. The human body's nutritional needs are pluralist. We need both iron and zinc, for example, but we can't trade them off in terms of some higher measure of nutritiveness. No amount of iron will make up for a zinc deficiency, or vice-versa. So if we were to find ourselves in a situation where we had fatal iron and zinc deficiencies, and we had the opportunity to take just one pill -- either an infinite iron supplement with no zinc, or an infinite zinc supplement with no iron -- there's no choice that would save us. But while my analogy here comes from science, the appeal of this form of pluralism is usually couched in humanities terms. No-win moral dilemmas are often framed as having a sort of tragic grandeur, more deep and noble than monistic philosophies that can crank out an answer to any moral dilemma you give them. I find this defense absurd. It seems to be a product of considering moral questions only as matters of abstract philosophizing, in which case insoluble dilemmas have a sort of interestingness and humbling blow-your-mind-ness. But as I see it, ethics is a pragmatic pursuit aimed at telling us how to act. While ethical systems may run into genuine problems, to praise the production of such problems as a virtue of an ethical system and denigrate other systems for giving answers is contrary to the whole purpose of ethics. It's as if Bill Gates started telling us that the blue screen of death is an important feature of Windows, and he feels sorry for those Mac users whose computers narrow-mindedly respond to their commands, depriving them of the tragic wonder of a system crash that destroys their data*.
*In my admittedly limited experience, I've actually had Macs crash about as often as Windows machines.