"We have learned how to jump off the speck of matter on which our species began; infinity, and therefore perhaps eternity, is being put into the hands of the human race. You cannot be so small-minded as to think that the rights or the life of an individual or of a million individuals are of the slightest importance in comparison with this."
This portion of the dialogue between Ransom and Weston reveals Weston's philosophy of utilitarian progress. To Weston, individual human life is worth being sacrificed for the large-scale progress of "humanity" as a whole. In the traditional utilitarian manner, he accuses Ransom of being small-minded to object to such a meager sacrifice as the lives of a few humans in the face of the progress of the species. Ransom, in answer, says, "I happen to disagree, and I always have disagreed, even about vivisection."
"[The lord of the silent world] has taught you to break all of them except [the love of kindred], which is not one of the greatest laws; this one he has bent till it becomes folly and has set it up, thus bent, to be a little, blind Oyarsa in your brain. And now you can do nothing but obey it, though if we ask you why it is a law you can give no other reason for it than for all the other and greater laws which it drives you to disobey."
In this section of the dialogue between Oyarsa and Weston near the end of the novel, Weston is giving a defense of his progressive, utilitarian worldview. Oyarsa, in response, is calmly and gently correcting him and demonstrating the faults in his system. This particular quote is explaining humanity's absurd devotion to "humanity" above all else, even the life of his neighbor. This vague ideal has become idolized in society, and there is no basis for pursuing its progress above any other progress at all - Oyarsa attributes this perversion to the "lord of the silent planet" (essentially Satan).
"A pint of bitter, please."
In this, the last spoken line of the plot, Lewis inserts a bit of humor while accentuating the incredible occurrences of the novel. Dr. Ransom has just been drugged and kidnapped for the purpose of becoming a human sacrifice to an alien race on Mars, before escaping and integrating himself with the cultures of speaking otters, giants, and other interesting creatures. On the terrifying return journey, his space-ship crashes and disappears along with his companions. He staggers into a bar and asks for a "pint of bitter," hilariously emphasizing the absurdity of his situation and the need for a well-deserved break.
