The Trivium was the real basis of the secular education of the period. Grammar included both the rules formulated by Donatus and Priscian and the study of a few of the classical writers of ancient Rome. "Under the head of rhetoric, the treatises of Cicero, such as the 'De Oratore' and the pseudo-Ciceronian 'Ad Herennium' were largely read. The elements of Roman Law were often added, and all schoolboys were exercised in writing prose or what passed for prose."[392] The most prominent and important of the subjects of secular instruction was Dialectic or Logic. The student of this subject had at his disposal richer material than in most other branches of secular knowledge. Rashdall instances the translations by Boethius of the "De Interpretatione" and the "Categoriae," as well as the "Isagoge" of Porphyry.[393] It was this concentration on Dialectic by minds whose chief interest was theology that paved the way to that philosophic system known as scholasticism. From its nature, it is scarcely possible to define scholasticism, but its meaning may be understood by considering the ground on which theological statements were based. For some centuries, such statements were required to be accepted merely on the authority of the Church. By the eleventh century, heretical views had crept in which could scarcely be dealt with so summarily. The stimulation of intellectual interests, due to the Crusades, made it necessary that theological beliefs should be carefully formulated and defended by intellectual weapons. The history of scholasticism falls into two fairly distinct periods. Among the great names associated with the first period are Anselm, "the last of the great monastic teachers," Roscellinus, William of Champeaux, and Abelard, "the true founder of the Scholastic Theology."[394] The second period which extended from the beginning of the thirteenth century to the Renaissance was the period of the culmination of scholastic thought and its consolidation into a system. The "great schoolmen" include Albertus Magnus, a Dominican who has been described as "the great organising intellect of the Middle Ages," Thomas Aquinas, famous as the scholar who brought scholasticism to its highest development by harmonising Aristotelianism with the doctrines of the Church, and two Englishmen, both of whom were Franciscan friars, Duns Scotus, and William of Occam.