Under what doctrine does an ethical society operate?

An ethical society operates under the moral doctrine of individualism — where each individual is regarded as an end to oneself, and not as a slave for the ends of others. This moral basis of Individualism is an expression of The Objectivist Ethics:

“The Objectivist ethics proudly advocates and upholds rational selfishness—which means: the values required for man’s survival qua man—which means: the values required for human survival—not the values produced by the desires, the emotions, the ‘aspirations,’ the feelings, the whims or the needs of irrational brutes, who have never outgrown the primordial practice of human sacrifices, have never discovered an industrial society and can conceive of no self-interest but that of grabbing the loot of the moment.”

“The Objectivist ethics holds that human good does not require human sacrifices and cannot be achieved by the sacrifice of anyone to anyone. It holds that the rational interests of men do not clash—that there is no conflict of interests among men who do not desire the unearned, who do not make sacrifices nor accept them, who deal with one another as traders, giving value for value.” — Ayn Rand, “The Objectivist Ethics”, VOS

Isn’t capitalism justified by the fact it serves the “public good”?

That capitalism serves the “public good” (defined as the sum of the good of all individuals) is true.

However, this is not capitalism’s moral justification but is merely an effect of its cause: by freeing the innovators to create and produce, capitalism results in a society where progress is the norm, and the standard of living for all (“the public”) rises.

“The moral justification of capitalism does not lie in the altruist claim that it represents the best way to achieve “the common good.” It is true that capitalism does—if that catch-phrase has any meaning—but this is merely a secondary consequence. The moral justification of capitalism lies in the fact that it is the only system consonant with man’s rational nature, that it protects man’s survival qua man, and that its ruling principle is: justice.” — AYN RAND

Where can I learn more about the moral basis of Capitalism?

Read Ayn Rand’s book The Virtue of Selfishness.

“A collection of essays that sets forth the moral principles of Objectivism, Ayn Rand’s controversial, groundbreaking philosophy.”

“Since their initial publication, Rand’s fictional works—Anthem, The Fountainhead, and Atlas Shrugged—have had a major impact on the intellectual scene. The underlying theme of her famous novels is her philosophy, a new morality—the ethics of rational self-interest—that offers a robust challenge to altruist-collectivist thought.”

“Known as Objectivism, her philosophy holds human life—the life proper to a rational being—as the standard of moral values and regards altruism as incompatible with man’s nature. In this series of essays, Rand asks why man needs morality in the first place, and arrives at an answer that redefines a new code of ethics based on the virtue of selfishness.”