In Moore’s defense of the primitiveness of “good”, he draws an analogy between the good and the yellow:
“My point is that ‘good’ is a simple notion, just as ‘yellow’ is a simple notion; that, just as you cannot, by any manner of means, explain to any one who does not already know it, what yellow is, so you cannot explain what good is.” (PE, 7)
What is interesting is not so much the truth of what Moore says as the questions it raises. One pertinent observation is that you can certainly explain what “yellow” means to someone who doesn’t already know it. One such potential explanation is:
(1) “Yellow” (in English) is synonymous with “amarillo” in Spanish.
Other explanations are not clearly either explanations of the meaning of “yellow” or explanations of what yellow is. Consider:
(2) Yellow is the color of objects such as ripe bananas and “Yield” signs.
Or, to give a different sort of explanation of what yellow is, consider:
(3) An object is (the color) yellow just in case it reflects light at a wavelength of roughly 597-577 nm.
In some sense, we could even “explain” (or, perhaps less controversially, teach) what yellow is to any reasonably intelligent pre-linguistic creature that perceives something like color. We could condition assorted blue (or, if you like, any non-yellow) stimuli with some aversive feedback and condition assorted yellow stimuli with some appetitive feedback. After a while, if the animal chose the yellow stimulus over the blue (or non-yellow) stimulus in every case, it would be clear that the animal could tell the difference between yellow things and non-yellow things. If, as seems reasonable, we take being able to tell the difference between yellow and non-yellow things as sufficient for knowing what yellow is, then we would have taught the animal what yellow is (if it didn’t already know). I once heard Marc Hauser describe such an experiment with bees (the colors were blue and red). Note also that this would be an explanation of what yellow is without being an explanation of what “yellow” means.
But note that, in every case, the explanation of what “yellow” means or what yellow is will require that some prior conditions obtain. In order for (1) to work, the student will have to know what “amarillo” means in Spanish. In order for (2) to work in the ordinary way, the student will have to know the color of ripe bananas and “Yield” signs. In that case, if the explanation works, the student will be able to distinguish yellow from non-yellow objects, as long as nothing weird happens to the lighting or her perceptual apparatus. In order for (2) to work in a less ordinary way, the student will have to know only, roughly, what ripe bananas and “Yield” signs are and what things are the same color as ripe bananas and “Yield” signs. In order for (3) to work, the student will have to know what it is to reflect light at 597-577 nm. In order for the explanation to the pre-linguistic creature to work, they will have to have whatever ability it is that enables them to be conditioned in the manner described. It might be objected against some of these “explanations” that the knowledge they yield of what yellow isn’t substantive enough, since they don’t require something like perceptual acquaintance with yellowness. If we want to say that to know what yellow is is to know what it is like to see yellow objects (which might be what Moore had in mind when he was referring to “yellow” as a “simple notion”), then we will have to admit that only some of these explanations will work, and only on certain conditions. In particular, (1) will work only if, so to speak, the student already knows what it is like to see “amarillo” objects; (2) will work only if the student knows what it is like to see (the colors of) ripe bananas and “Yield” signs; and (3) won’t typically work at all. I agree with Moore that the concept of a “simple notion” is probably best explicated or understood in terms of how it can be explained. If yellow is a simple notion, then if a notion can be explained in just the same ways as yellow, then that notion is simple.
Then the interesting question is, To what extent does the analogy between “good” and “yellow” hold? If the analogy holds all the way – if what goodness is can be explained only in ways precisely parallel to the ways in which what yellow is can be explained – then I think we have to say something like the following.
We can explain what good is or what “good” means to a student in a number of different ways. We can certainly give her a synonym (as in (1) above). Alternatively, we can give her something of the form:
(2`) Goodness is the F of objects such as x, y…
Here, we would have to substitute for “F” some relevant second-order property of goodness (just as color is the relevant second-order property of yellow), and substitute for “x”, “y”, and so forth a sequence of descriptions of sufficiently diverse good objects, actions, or states of affairs. What would make it “sufficiently diverse” would depend on how effective it is in getting the student to understand what goodness is, and also on one’s preferred understanding of knowing what goodness is. Again, we could have some explanation of the form:
(3`) x is good just in case p.
Here we would substitute for p something that holds whenever the substitution instance of x refers to something good.
Obviously, given that the “explanation by synonymy” will only hold in somewhat trivial cases, the difficulty in claiming that the analogy holds in full is in specifying, or at least giving an existence proof, of the relevant values of the variables in (2`) and (3`). For instance, in the explanation of what yellow is in (2), the description of yellow as a color is doing a lot of work. It is not at all clear that there is any second-order property of goodness that could do a similar job in an explanation of the form (2`). And for explanations of the form (3`), it is clear that any substitution-instance for p will be controversial, if at all plausible.
On the other hand, if the analogy fails, we’ll need some good explanation of why it fails. Also, if these explanations of what good is don’t work, then we’ll need some story about how children come to understand what good is, or, failing that, what “good” means, which we do not yet have.