Désiré Joseph Mercier, a Thomist philosopher and Roman Catholic cardinal, was born in the Walloon section of Brabant, Belgium. At the end of his secondary education, Mercier decided to study for the priesthood; he studied philosophy and theology at the Malines Seminary for five years and subsequently at the University of Louvain. Ordained in 1874, he received the licentiate (equivalent to the current doctorate) in theology in 1877. The same year he was named professor of philosophy at the Malines Seminary, where he taught logic and psychology for the next five years.

The famous encyclical, Aeterni Patris, of Pope Leo XIII, urging the restoration of scholastic, particularly Thomistic, philosophy, was published in 1879. In 1882 a chair of Thomistic philosophy was established at Louvain, and Mercier was named to this post.

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