In May, Pope Francis released his historic encyclical, Laudatio Si, or “Praise Be.” He chose his papal name, he explains in the text, because he considers St. Francis of Assisi to be “the example par excellence of care for the vulnerable and of an integral ecology lived out joyfully and authentically.” His namesake showed that concern for nature is inseparable from justice for the poor, social commitment, and peace within oneself.

The encyclical’s title refers to the Canticle of the Sun, St. Francis’s song of praise to God for all creation -- the foremost expression of environmental holism within the Roman Catholic tradition. Yet the Canticle’s praise for “Brother Sun” and “Sister Moon” has seemed so close to nature-worship that some have doubted whether it could ever be included in mainstream Catholic thinking.

Those doubts have now been laid to rest. Francis’ predecessor, Pope Benedict XVI, began to turn the Church’s attention toward the need for environmental sustainability. Francis has taken this process much further.

Laudatio Si has received considerable media attention, most of it focused on its uncompromising call for action on climate change. That is appropriate, for it is of the utmost importance that the leader of the world’s 1.2 billion Roman Catholics has unequivocally stated that scientific studies attribute “most global warming” in recent decades to greenhouse gases, “released mainly as a result of human activity.”

There is, however, another aspect of the encyclical that has received much less attention. St. Francis is among the most popular Catholic saints because of his reputation as a friend of animals. In keeping with that tradition, Laudatio Si amounts to the strongest statement against harming animals ever made by a pope in a document as authoritative as an encyclical.

Mainstream Christian thinking about animals is rooted in the Book of Genesis, where God is said to have granted man dominion over all the animals. St. Thomas Aquinas interpreted that verse as implying that it simply does not matter how man behaves toward animals; the only reason why we should not inflict whatever cruelties we like on animals is that doing so may lead to cruelty to humans.

A few Christian thinkers have sought to reinterpret “dominion” as “stewardship,” suggesting that God entrusted humanity to care for his creation. But it remained a minority view, favored by environmentalists and animal protectionists, and Aquinas’s interpretation remained the prevailing Catholic doctrine until the late twentieth century.

Francis has now come down decisively against the mainstream view, saying that Christians “have at times incorrectly interpreted the Scriptures,” and insisting that “we must forcefully reject the notion that our being created in God’s image and given dominion over the earth justifies absolute domination over other creatures.” Our “dominion” over the universe, he declares, should be understood “in the sense of responsible stewardship.”

Against the background of nearly 2,000 years of Catholic thinking about “man’s dominion,” this is a revolutionary change. But the encyclical includes another statement that could have even more far-reaching implications. That statement, which originally appeared in the Catechism of the Catholic Church issued by Pope John Paul II in 1992, calls it “contrary to human dignity to cause animals to suffer or die needlessly.” To ensure that the sentiment would be noticed, Francis tweeted it. (Yes, Francis tweets, using the Twitter handle @Pontifex.)

When is suffering and death “needless”? If you can nourish yourself adequately without eating meat, isn’t buying meat needlessly causing, or at least being complicit in causing, the death of an animal? Isn’t buying eggs from hens who have led a miserable life, jammed into small wire cages, needlessly causing, or being complicit in causing, the suffering of animals?

Before Cardinal Ratzinger became Pope Benedict XVI, he gave an interview in which he deplored the “industrial use of creatures” such as hens living “so packed together that they become just caricatures of birds.” Unfortunately, right now, tens of billions of chickens are being forced to live this way; indeed, mankind’s realm is full of unnecessarily suffering animals.

Although animal advocates implored Ratzinger to reiterate his views on animal welfare after he became Pope, he never did so. Francis, by contrast, appears to have been referring to factory-farmed animals when he spoke, in The Joy of the Gospel, of “weak and defenseless beings who are frequently at the mercy of economic interests or indiscriminate exploitation.”

Now, in Laudatio Si, Francis quotes the passage in the Gospel of Luke, in which Jesus says of the birds that “not one of them is forgotten before God.” Francis then asks: “How then can we possibly mistreat them or cause them harm?” It is a good question, because we do mistreat them, and on a massive scale.

Most Roman Catholics participate in this mistreatment, a few by raising chickens, ducks, and turkeys in ways that maximize profit by reducing animal welfare, and the majority by buying the products of factory farms. If Pope Francis can change that, he will, in my view, have done more good than any other pope in recent history.

By Peter Singer, professor of bioethics at Princeton University and laureate professor at the University of Melbourne