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The French theologian Peter Abelard was possibly the most brilliant thinker of the twelfth century. He studied under Roscelin (d. c. 1125), the nominalist, who believed all universalia is pure mental conception. Later he studied under William of Champeaux (c. 1070-1121), a realist, who believed that universalia is the essence of all existence. Opposing his teachers to find a middle position, Abelard saw universalia as a mental concept existing not independently from individuals but also not as arbitrary mental concepts.