For many years, I have been entranced with the puzzle that is quintessential to meta-ethical philosophy: are moral values in any way objective and absolute? if so, how do we know, and if not, why do they often feel as if they are?
After much graduate-school (and beyond) deliberation, the "solution" I have been most comfortable with is that of error theory. In a nutshell, error theory tells us that moral values are subjective and relative (rather tahn objective and absolute), and that we easily fall into error because they are felt so strongly as to appear objective. The most famous expounder of error theory (I think, the pioneer of the name) was JL Mackie, though shades of error theory can be found in earlier philosophers like Bertrand Russell, William James, and David Hume.
As error theory is a form of relativism, it is best to briefly justify relativism and only then, bring up reasons why error theory is the best explanation of our moral climate.
Why am I a relativist? Before even getting into meaty philosophical argument, I am a relativist because of the very existence of widespread moral disagreement. Moral objectivists argue that for ethical questions, there exists one right answer (whether it is discoverable by reason or an intuitive moral sense that we all share). Were this true, then the only explanation for the fact of widespread moral disagreement on issues would be that only some of us are attuned enough (in reason or intuition) to recognize the right answer. (Generally, when moral objectivists are presented with the fact of moral disagreement, their answer to it IS in fact to poinit out that those who do not arrive at a particular answer are simply wrong).
While I guess this is plausible, there is a better solution: that there often exists more than one possible "right" answer to a question (pluralism) or, further still, that the very idea of what the "right" answer is, is a question that each individual judges for themselves based on their own sentiments, experience, and reasonings.
Why go for relativism over pluralism? This is where we get into moral psychology. I am a relativist because, as an epistemic fact, each of us has direct mental access only to our own brains. In addition, as best as we can tell, none of us has direct access to any "ethical plain" that exists outside of our brains. In some sense, this is what led to JL Mackie's idea, that undergirds error theory, called The Argument
From Queerness. The Argument from Queerness states that ethical propositions cannot be metaphysically "out there" in the world because they do not in any way appear to be natural properties. Moral debates are not like debates over concretes, as we cannot "point to" in order to show that a moral act is inherently right or wrong.
Moral objectivists will tend to reply that this is a scientism of the type which says that nothing exists which is not concrete. In a sense, they are correct. For someone to say that morality exists objectively would mean that they need to show how moral disputes can be objectively (rather than subjectively) resolved; what can be pointed to that is outside of the arguer's own moral sentiments that can prove to someone with different moral sentiments that "x is true." Often, the only avenue for the objectivist here is to suggset that "good" is an obvious quality (like "red" or "hot"). Here, it is easily pointed out that, unlike "red" and "hot," "good" cannot be experienced with any of the five senses, but rather "felt" with the moral intuition (bringing me back to the same suspicion that "good" and "bad" are judgments made differently by different people's intutions).
In the absence of any really good reason to see moral judgments as anything but subjective appraisals, or accompoanying arguments that human mind has some sort of direct access to anything outside of its own thoughts, I see no good choice but to suspect that subjective relativism is the most accurate description of how things are.
But why is this so hard for people to accept? Even though relativism seesm the most logical description, it is hard to actually be a moral subjectivist/relativist but in theory. We hold moral precepts very strongly and it is hard to think that they are simply subjective preferences. When I say that slavery, sodomy, murder, rape, etc., are wrong, I feel like they really are wrong and not just that I think them wrong.
This is why I think that error theory - the idea that, at times, our psychological natures make us experience our own preferences so strongly that it is impossible to imagine that they are not objective - is the best description of how things really are. Imagine the best piece of music you have ever heard in your entire life. At the apex of its enjoyment, isn't it just well nigh impossible to imagine that some people might derive no pleasure from it at all? Another common example is our senses of humor. It is probably tempting to suggest that those who do not laugh at the funniest joke you have ever heard are those who "have no sense of humor," rather than simply not sharing your sense of humor.
That is the way, I think, human psychology often works. When we deeply feel certain preferences, our brain presents these to us not as prefernces, but as imperatives that simply are (inependently of us feeling or thinking them so).
So, we can either see moral disagreement as a sign that some people are simply not good at moral reasoning or reading their moral intuitions (leading them into wrong answers), or suspect that people differ in moral sentiments becuase, like aesthetic sentiments, moral sentiments are not objective. We can either suggest that the reason people feel strongly about their moral opinions is because these opinions are objective (while ignoring that even "wrong" opinons are felt as strongly as "right" ones), or we can suspect that human psychology often "tricks" us into feeling sentiments so strongly that they appear, but are not, objective.
Quite simply, I think the latter options - subjective relativism and error theory - are the best interpretation of the facts of our moral universe.