These two episodes cover some related approaches in 20th century ethics:

First, we read Chapter 1 of G.E. Moore's Principia Ethica (1903), which argues against utilitarianism and other ethical philosophies by exposing the "naturalistic fallacy," which equates "good" with some natural property like pleasure or people's actual desires. This error, says Moore, also extends to equating good with what God wants or what we would choose upon calm reflection on social norms and our own innermost desires. It may well be that the good coincides with one of these categories, but that's not what the word "good" means, as it's always a sensible question to ask "but is pleasure good?" or "is God's will good?" for any alleged equivalent. No, says, Moore, good is a basic, indefinable, non-natural quality of the world. Buy the book or read it online. You can also listen to it.

We also read C.L. Stevenson's essay "The Emotive Meaking of Ethical Terms." (1937) A student of Moore as well as Wittgenstein, Stevenson argued that Moore was right in saying that "good" is indefinable by other terms like "pleasure," but disagreed with the claim that it picked out some basic object in the world. Rather, saying something is good is an act of advocating it. "Ethical terms are instruments used in the complicated interplay and readjustment of human interests." This position gives up on moral realism altogether, and like Wittgenstein, he gives an analysis of meanings of words as involving their use: saying something is good is a lot like saying "we like it," but "good" has a subtle emotive connotation built into it such that saying that something is good recommends that you like it too. Buy a compilation with this essay in it or read the essay online.

The narrative tying these episodes together Alasdair MacIntyre's After Virtue: A Study in Moral Theory (1981). For episode 58, we read just the first two chapters, where he critiques Moore and Stevenson, and moreso the historical climate that they exemplify and helped to bring to its current sorry state. For episode 59, we all read chapters 3-7 and 14-17 to further elaborate this critique as well his historical picture of the evolution of moral thought from Aristotle to the present and his picture of a modern moral realism based, like Aristotle's, on some sort of teleology, which is about referring to some sort of purposefulness built into human nature, or in any case humans as we find ourselves embedded in a particular culture, family, projects, and intellectual traditions/habits.

MacIntyre sees Stevenson's position as a reasonable response to Moore's given the observation that Moore's actual method of choosing what is morally good is a matter of baseless intuition, not reproducible across cultures or time periods. MacIntyre sees historical philosophers' critiques against Kant, utilitarianism, and intuitionism of all stripes as effective, but thinks that these Enlightenment thinkers don't adequately ground their proposed alternatives. He diagnoses the problem in the history of the concepts of nature, fact, and normativity: The fact/value distinction as it evolved through Enlightenment philosophy uprooted any chance of rational grounding for ethics. However, says MacIntyre, contrary to what we've been taught, it's not a logical rule that "ought" can't follow from "is."

For instance, here's a premise: I am a doctor. A conclusion deducible from this is: I ought to do what a doctor does. The concept of doctor is teleological; it has a normative component built into it, and to try to say that the concept is analyzable into its teleological/normative and its descriptive parts is to not really understand what a doctor is.

So, both Aristotle and and modern philosophers like Hume use human nature as a foundation for ethics, but the modern view of human nature is divorced from teleology (based on what to MacIntyre is a sensible rejection of Aristotle's theories of biology), and in fact the morality that we modern folks inherited and try to prop up is not really an outgrowth of an examination of this human nature, and it has produced moral recommendations that we really can't live up to and end up being useless to us (e.g. the semi-arbitrary taboos of a given culture getting encoded as morality). For Aristotle, part of the conception of human nature is an extra-natural idea of human excellence, and MacIntyre thinks that this is the crucial element for sensible moral thinking that has been lost to history, leaving our current moral discourse very confused. Buy the book. or here's a copy I found online.