Philosophical skepticism comes in many varieties. The skeptic can be a real challenger or a fictitious construct, created as a methodological tool in epistemology. Usually, the main similarity between all forms of skepticism is that it concerns the epistemic realm; however, in meta-ethics, there are two kinds of skeptical challenge that can be raised. Unlike other areas of philosophical inquiry, one can admit that we have moral knowledge, or that moral knowledge is possible, while still remaining a moral skeptic in completely different sense. Moral skepticism comes in a practical variety as well, which is best characterized by the question, “why be moral?”.
Call the two different kinds of moral skepticism, epistemic moral skepticism and practical moral skepticism. The former is analogous to the traditional skepticism that targets the epistemic realm of justification and knowledge, whereas the latter targets reasons for action. The practical moral skeptic questions why moral reasons ought to move us in any way. The epistemic moral skeptic will raise well known structural challenges, such as the regress problem, as well as challenges concerning how we distinguish true from false representations or impressions, and challenges stemming from (allegedly) possible skeptical scenarios like the evil demon and brains in vats. Typically, the antiskeptic can appropriate strategies used against more general kinds of skeptics in epistemology. However, some people in meta-ethics think ethics has distinct (epistemic) skeptical challenges that aren’t found elsewhere, such as concerns arising from intractable (in principle) moral disagreement.
A good way of representing the practical skeptic is as the amoralist who is unmoved by ethical concerns. What could we give the amoralist in terms of reasons that would convince him to be moral? The amoralist will reject moral reasons, so one typical way of meeting the challenge is by providing selfish reasons to be moral, such as rational self interest over long-term interpersonal interactions. However, there are instances where we would want the amoralist to act morally even though there aren’t sufficient selfish reasons to do so, which is made salient by the Ring of Gyges. Imagine somebody had a ring that could make them undetectable when performing actions. Modify the situation to make that somebody an amoralist, and then ask what reasons we could provide him to convince him to act morally when wearing the ring.
Another strategy for countering the amoralist is a position called internalism. I have explained the varieties of internalism in a previous post, so I’ll briefly outline the relevant elements here rather than rehashing entire positions. If one takes the position that recognizing moral facts necessarily provides the recognizer with reasons for action, then the amoralist (assuming he has moral knowledge) will be impossible. Another position is internalism about moral judgment, which says that anybody who makes a sincere moral judgment necessarily is (at least partially) motivated to act morally, which means that an amoralist who makes sincere moral judgments is impossible. If an amoralist cannot make sincere moral judgments, then he is in some way deficient, and not a suitable example for raising challenges concerning practical moral skepticism. The amoralist will be so unlike normal people that he won’t be capable of using moral concepts, which means that he does not raise genuine concerns about moral reasons or motivation. To be a challenge, he would have to employ the same moral concepts we do, and competently so. It would be like raising a challenge to the claim that pain provides reasons for action by producing a thought experiment concerning a being that cannot feel pain.
An externalist, on the other hand, can admit that genuine amoralists are possible, and not deficient in any relevant way. The externalist will simply say that normal humans operate under psychological laws that reliably link up recognition of moral facts with motivation to act morally. The amoralist will be a rational actor who happens not to operate under such psychological laws. If the externalist is also a moral realist, then the amoralist will be said to be both rational and morally reprehensible (if he acts immorally). Rationality and morality are not as tightly connected on most externalist theories as they are on many internalist theories. Internalism tends to be an element of moral rationalism, which takes moral rationality to be a species of practical rationality. Moral rationalism entails that amoralists are practically irrational in some way (which means rational amoralists are impossible). Externalists tend to take a Humean theory of rationality, which means that one is practically rational just if one’s actions align with one’s desires. So, according to the Humean, the amoralist merely has a different desire-set than normal people, which means that the amoralist acts rationally by not being moved by moral concerns, whereas normal people operating under normal psychological laws would be irrational if they weren’t moved by moral concerns.
Practical moral skepticism is a unique form of skepticism, as it concerns action rather than belief. The challenge could be extended to standards of rationality in general, insofar as they concern rational action rather than (just) belief. However, that is a topic for a different occasion.