[http://en.wikipedia.org/wiki/Pope_Leo_XIII]
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Rerum Novarum
His encyclicals changed Church positions on relations with temporal authorities, and, in the 1891 encyclical Rerum Novarum addressed for the first time social inequality and social justice issues with Papal authority, focusing on the rights and duties of capital and labour. He was greatly influenced by Wilhelm Emmanuel von Ketteler, a German bishop who openly propagated siding with the suffering working classes in his book Die Arbeiterfrage und das Chistentum.
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