During the 11th century, developments in philosophy and theology led to increased intellectual activity. There was debate between the realists and the nominalists over the concept of "universals". Philosophical discourse was stimulated by the rediscovery of Aristotle and his emphasis on empiricism and rationalism. Scholars such as Peter Abelard (d. 1142) and Peter Lombard (d. 1164) introduced Aristotelian logic into theology. In the late 11th and early 12th centuries cathedral schools spread throughout Western Europe, signalling the shift of learning from monasteries to cathedrals and towns. Cathedral schools were in turn replaced by the universities established in major European cities. Philosophy and theology fused in scholasticism, an attempt by 12th- and 13th-century scholars to reconcile authoritative texts, most notably Aristotle and the Bible. This movement tried to employ a systemic approach to truth and reason and culminated in the thought of Thomas Aquinas (d. 1274), who wrote the Summa Theologica, or Summary of Theology.