Moore makes a strange assertion in this book. A “simple,” as in, non-composite thing cannot have a definition.
He considers the definition of a horse, saying that it “is composed in a certain manner: that it has four legs, a head, a heart, a liver, etc., etc., all of them arranged in definite relations to one another. It is in this sense that I deny good to be definable.” Outrageous! Suppose that good is indeed simple, as in having no components of which it might be made up. That only means that it lacks a material cause. It says nothing about its formal cause!
The former is an answer to the question, “What parts does good consist of?” The latter, to the question “What is good or the good?” These questions inquire of very different things, and conceding that materially, good is “nothing” does not entail that it is “nothing” formally, as well.
Let me illustrate the distinction.
Peruse the Wikipedia entry on the DNA. The first sentence is that it’s “a molecule that encodes the genetic instructions used in the development and functioning of all known living organisms and many viruses.” That sounds interesting.
The rest of the article, unfortunately, busies itself with the question of how the DNA is constructed, and a bit on how it works. But I, in turning to Wikipedia, want to know first and foremost what the DNA is! What does it do, and why? How does it further the ends of organisms, from their mere survival all the way up to pursuit of happiness, which pertain also to its final cause? Why is the DNA used in the “development and functioning of all known living organisms” as opposed to some other structure that might exceed it in efficiency? Given almost complete lack of attention devoted to such primal questions, the article is a failure.
Moore then uses the comparison of “good” to “yellow,” saying that “yellow,” too, is simple and (therefore) undefined. This, too, is nonsense. Yellow is “undefined,” because it is a personal subjective experience, and those are fully private and incommunicable. How do I describe how yellow feels like to my sight? I cannot. How can I make sure that my experience of yellow is the same as your experience? I cannot do that, either, if that question is even meaningful. As a result, “yellow” cannot be defined other than by different types of ostention and hoping that our human bodies work sufficiently similarly that, however the mind-body connection is effected, the spiritual experiences are close to each other in “quality,” as well, whatever exactly that means.
Presumably, however, Moore does not hold that good, just like yellow, is a subjective experience. The analogy fails. Let’s hope, as I continue re-reading this book, that he will try to prove the simplicity of goodness in some other way.