Vatican City (CNN) -- A pope not only should, but must resign if he is not able to carry out his duties, Pope Benedict XVI says in a wide-ranging new book out Tuesday.
If the head of the Roman Catholic Church is "physically, psychologically or spiritually" unable to carry out his duties, he has not just a right but an obligation to step down, the pope says.
No pope has resigned in nearly 600 years.
Benedict is 83 years old and apparently in good health.
His predecessor, John Paul II, served until his death despite becoming so physically frail in his later years that it was difficult to hear him speak.
Benedict's comments on resignation come in a book which has been making headlines for its revelation that the pope may consider the use of condoms morally justified in some circumstances.
He says there may be a case for prostitutes to use them to prevent the spread of HIV and AIDS.
That announcement -- which does not rise to the level of official Catholic policy -- was a shock because the Vatican forbids any use of artificial contraception.
"There could be single cases that can be justified, for instance when a prostitute uses a condom, and this can be a first step towards a moralization, a first assumption of responsibility, to develop again the awareness of the fact that not all is allowed and that one cannot do everything one wants," Benedict says in the book, "Light of the World: The Pope, the Church, and the Signs of the Times."
"However, this is not the true and proper way to defeat the infection of HIV," he added, saying the "humanization of sexuality" is the best method.
There has been debate about whether the pope was talking about male or female prostitutes, since the German and Italian translations of the book use different genders for the word.
The Vatican's official spokesman said Tuesday it doesn't matter.
"If a man, or a woman, or a transsexual does it, it is all the same," the Rev. Federico Lombardi said, adding he had consulted the pope about the question. "The point is that a first step of responsibility is being taken to avoid a grave risk for another person."
The author of the book about Benedict voiced a certain frustration about the focus on condoms over the past few days.
"The world is coming to an end but the only interest is in condoms," Peter Seewald said.
Benedict is more concerned about environmental disaster, the German journalist and author said.
"The pope ... says we cannot afford to continue living as we have done before," Seewald said in a press conference at the Vatican.
"He provides answers about what society in the future should look like," the writer said. "He says what we can change and must change to guarantee human life survives on this planet."
The 239-page book is taken from a weeklong series of interviews Seewald had with the pope, the author said. Seewald had twice before interviewed the pope, when he was Cardinal Joseph Ratzinger, and the interviews resulted in two best-sellers.
"Light of the World" is due to hit bookstores Wednesday.
The United Nations program on HIV and AIDS welcomed the pope's comments on condoms and said they could have a huge impact on stopping the spread of the disease.
The pope caused a firestorm of protest last year when he spoke out against condoms as a way of controlling AIDS during a trip to Africa, the continent hardest hit by the disease.
Other topics in the book include the clerical abuse scandal, dialogue with Islam, homosexuality in the priesthood, and the global climate crisis.
While some may see the book as a change in church thinking, Vatican watchers say it simply reveals the nuanced thinking of the pope in confronting modern challenges.
"He uses a lot of language -- very direct, very stark language -- that I think will surprise people on contraception, on homosexuality, on issues where the church, if you like, and politically correct society are very far apart," said Austen Ivereigh of Catholic Voices, a group of Catholics who speak about church policy. "But is this an unprecedented event that we have an interview like this? The answer is yes."
CNN's Hada Messia, Nadine Schmidt and Atika Shubert contributed to this report.