Thomism is the expression applied since the fourteenth century to philosophers whose thinking has its foundations in the thought of St Thomas Aquinas. Neo-Thomism is usually understood to refer to the revival of Thomism which began in the middle of the nineteenth century and was later officially endorsed by the Roman Catholic Church. It was initiated by Italian professors of philosophy who were dissatisfied with early nineteenth century Italian philosophy and the traditionalist reaction to it. They called for a revival of the study of the philosophy of Thomas. Their aim was not a mere restatement of Thomas' philosophy, but an accurate understanding of the permanent validity of the principles of Thomas' philosophy that could be applied to contemporary problems. Although initially in conflict with some leaders of the Church, the movement gained momentum and support from the Church towards the latter part of the nineteenth century. Pope Leo XIII's encyclical, Aeterni Patris (1879), called the Church to a study of philosophy and in particular a study of the doctrines of Thomas Aquinas as a means of addressing contemporary needs.