There are many skeptical arguments that try to undermine hopes for moral wisdom. Here is a perennial favorite, a variation on an argument developed by the brilliant Scottish philosopher David Hume (1711-1776). Let's call this Hume's Argument, in his honor:
1. We can know only two sorts of claims: conceptual truths or empirical truths.
2. Moral claims are neither conceptual truths nor empirical truths.
3. Therefore, we can have no moral knowledge....
Hume had a supporting reason for thinking that moral knowledge could not be mpirical. Empirical knowledge tells us how to describe the world. And when we describe the world, we talk about what is the case. But morality speaks of what ought to be the case. How can we get from descriptions to prescriptions? How does knowing how the world actually works enable us to learn how it ought to work? Hume thought that there was no answer to this question. If he is right, there is a gap between what is and what ought to be, a gap that can never be crossed.
One hardly knows where to begin. I suppose one has to begin with what Hume's actual view is. Hume thinks moral claims can be empirical truths. Shafer-Landau footnotes the Treatise of Human Nature, Book III, Part 1 as the source of "Hume's Argument"; but Book III, Part 1 is specifically about rationalism. In Shafer-Landau's terminology, it argues that morality is not a conceptual truth (more precisely, it is not a relation of ideas recognized by reason). If one reads just a little more, into Part 2, one quickly discovers that Hume is a moral sense theorist, and thus in Shafer-Landau's terminology holds that morality consists of empirical truths about moral sentiment. This is the very first paragraph of Part 2:
Thus the course of the argument leads us to conclude, that since vice and virtue are not discoverable merely by reason, or the comparison of ideas, it must be by means of some impression or sentiment they occasion, that we are able to mark the difference betwixt them. Our decisions concerning moral rectitude and depravity are evidently perceptions; and as all perceptions are either impressions or ideas, the exclusion of the one is a convincing argument for the other. Morality, therefore, is more properly felt than judged of; though this feeling or sentiment is commonly so soft and gentle, that we are apt to confound it with an idea, according to our common custom of taking all things for the same, which have any near resemblance to each other.
He then goes on to talk about how moral distinctions are founded on sensations, namely, some kind of pleasure and pain (but not every kind of pleasure or pain). This is as empirical as it gets; it would have been recognized as obviously such in Hume's day, and Hume's own empiricism requires taking it as such.
As to the claim that you can't get an ought from an is, I have discussed this before, so I will simply link to some of my posts on it:
Hume on Ought and Is, Part I: Background
Hume on Ought and Is, Part II: The Argument
Hume on Ought and Is, Part III: Conclusions
The irony is that Hume is being deployed in context against natural law theorists on one of a very small handful of things with respect to which Hume agrees with natural law theorists: that we are naturally capable of recognizing moral good and bad.
Fortunately, disagreeing with the book sometimes makes for the best philosophy classes.