Pope Francis’ encyclical on climate change cited many of the usual sources: the Bible, his predecessors in the Vatican and his namesake, Saint Francis of Assisi. It also cites ninth century mystical Muslim poet Ali-al-Khawas.

In the sixth chapter of the nearly 200-page papal letter, Francis writes that humanity can “discover God in all things.”

“The universe unfolds in God, who fills it completely. Hence, there is a mystical meaning to be found in a leaf, in a mountain trail, in a dewdrop, in a poor person’s face,” the pope writes.

In a footnote to that quote, he credits al-Khawas for the concept of nature’s “mystical meaning,” noting how the poet stressed “the need not to put too much distance between the creatures of the world and the interior experience of God.”

He then directly quotes the poet: “The initiate will capture what is being said when the wind blows, the trees sway, water flows, flies buzz, doors creak, birds sing, or in the sound of strings or flutes, the sighs of the sick, the groans of the afflicted.”

Alexander Knysh, a professor of Islamic studies at the University of Michigan, said that the idea Pope Francis is drawing on in this passage has been influential in literature, including Western figures such as English Romantic poet William Blake.

“According to (the idea), God actively and constantly reminds his servants about his immanent presence not just by means of various phenomena but also by various sounds and noises—rustling of leaves, thunder, rainfall,” Knysh says.

It’s unusual for a pope to cite a Sufi poet, but those who have known Francis since his days in the slums of Argentina say that shows his personal touch on the encyclical.

“He’s trying to foster ecumenical and interfaith dialogue about shared spirituality,” Father Augusto Zampini, an Argentinian priest and theological advisor to the Catholic Agency For Overseas Development, tells TIME.

“He’s inviting all human beings to transcend, to go out of themselves and therefore to improve the relationship that we have with our people, with the Earth, with God.”

Read Next: Pope Francis Urges Climate-Change Action in Encyclical

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