Thomism is the philosophical school that arose as a legacy of the work and thought of St. Thomas Aquinas. The word comes from the name of its originator, whose Summa Theologica was one of the most influential documents in medieval philosophy and continues to be studied today in philosophy classes. In the encyclical Doctoris Angelici, Pope St. Pius X cautioned that the teachings of the Church cannot be understood scientifically without the basic philosophical underpinnings of Aquinas' major thesis: The Second Vatican Council described Aquinas' system as the "Perennial Philosophy."