For those unfamiliar with the story, Malancandra is Lewis' vision of an unfallen, Edenic planet Mars where several races of rational creatures exist in harmony under the rule of an angelic being called Oyarsa. Weston and Devine are humans bent on using Malacandra for their own purposes, and Ransom - the protagonist - is taken to the planet against his will to help them achieve their ends.
Devine's interest in Malacandra is simply profit (it's loaded with gold), but Weston is driven by loftier ideals. Knowing that Earth can only sustain humanity for a limited period of time, Weston sees Malacandra as an opportunity to extend the human genetic line indefinitely. This is his summum bonum, and he is committed to it even if it means the devastation and eventual extinction of the Malacandrians.
In my favorite scene, Weston tries to justify his behavior to Oyarsa, but his limited knowledge of the Malacandrian language makes it necessary for Ransom to translate. This is very difficult for Ransom to do, because the Malacandrians lack familiarity with evil. For example, Weston begins his speech by saying, "To you I may seem a vulgar robber...," not realizing there aren't any robbers in Malacandria, nor any word for robber. Ransom, then, has to explain the concept of stealing: "Among us, Oyarsa, there is a kind of [person] who will take other [person's] food and--and things, when they are not looking. He says he is not an ordinary one of that kind."
As Weston speaks, Ransom has to act as a bridge between the corrupted and the pure, and his job becomes increasingly difficult as Weston tries to explain his ethical views. It's a great scene where the absurdity of trying to deduce moral values from naturalism is made abundantly clear. Weston argues that humans have the right to destroy the Malacandrians because they are more advanced. "Our right to supersede you is the right of the higher over the lower," he says. "Life is greater than any system of morality; her claims are absolute. It is not by tribal taboos and copy-book maxims that she has pursued her relentless march from the amoeba to man and from man to civilization." The absurdity of these words is made evident as Ransom attempts - unsuccessfully - to express what Weston is saying:
"He says," began Ransom, "that living creatures are stronger than the question whether an act is bent or good--no, that cannot be right--he says it is better to be alive and bent than to be dead--no--he says--I cannot say what he says, Oyarsa, in your language. But he goes on to say that the only good thing is that there should be very many creatures alive. He says there were many other animals before the first men and the later ones were better than the earlier ones; but he says the animals were not born because of what is said to the young about bent and good action by their elders. And he says these animals did not feel any pity."The reason Ransom can't express what Weston is saying is because he is guilty of the "is-ought fallacy." Weston observes that, in nature, stronger life forms survive while weaker forms die out. Since he lacks any basis for morals apart from nature, he assumes that whatever the natural course of nature is is synonymous with what ought to be. In this case, that means that humans have the right to overthrow the Malacandrians. But what Weston fails to realize is that what is and what ought are not necessarily synonymous. Even if Weston is right about the evolutionarily process, the evolutionary process is silent about what ought to be.
Weston says that what ought to be is what nature does, and that should take precedence over any other moral system. But if what ought is determined by what is, then everything that is ought to be. If everything that is ought to be, we might as well get rid of the category of oughtness entirely, because there is no such thing as not-ought.
Weston says, "Life is greater than any system of morality; her claims are absolute." A statement like this might sound meaningful on the surface, but a closer examination reveals that it can only be received by corrupted ears. In what sense is life "greater" than any system of morality? Is the greatness of a person or action solely determined by its power? If so, then perhaps life is "greater than any system of morality." But greatness is never defined this way. Something is not great simply because it is powerful, but because it is good and right. Greatness, therefore, is oughtness combined with is-ness. But if this is how we view greatness, how can something be great if it disregards every system of morality? It can't. To disregard every system of morality is to eliminate the category of oughtness, and eliminating the category of oughtness renders the concept of greatness meaningless. This is why Ransom stumbles over his words.
"Life is greater than any system of morality," Weston says. Does this mean that "living creatures are stronger than the question of whether an act is bent or good"? Only if we reduce greatness to nothing more than power, and that is not what we mean when we talk about greatness. Does it mean that "it is better to be alive and bent than to be dead"? Only if we say that the concept of betterness has nothing to do with oughtness, but if we remove the concept of oughtness from betterness we empty the concept of its meaning.
In short, Weston's words can't be translated for uncorrupted ears because they are nonsense. Unfortunately, attempts to derive oughts from is-es are just as common today (or perhaps more so) as they were in Lewis' day. The good news, though, is that Lewis' allegory is just as relevant now as it was then. Good stuff.