In composite substances we find form and matter, as in man there are soul and body. We cannot say, however, that either of these is the essence of the thing. That matter alone is not the essence of the thing is clear, for it is through its essence that a thing is knowable and is placed in a species or genus…

A small error at the outset can lead to great errors in the final conclusions, as the Philosopher says in I De Caelo et Mundo cap. 5 (271b8-13), and thus, since being and essence are the things first conceived of by the intellect, as Avicenna says in Metaphysicae I, cap. 6, in order to avoid errors arising from ignorance about these two things, we should resolve the difficulties surrounding them by explaining what the terms being and essence each signify and by showing how each may be found in various things and how each is related to the logical intentions of genus, species, and difference.

Thomas Aquinas, OP ( 1225 – 7 March 1274), also Thomas of Aquin or Aquino, was an Italian Dominican friar and priest and an immensely influential philosopher and theologian in the tradition of scholasticism, within which he is also known as the "Doctor Angelicus", "Doctor Communis", and "Doctor Universalis". "Aquinas" is from the county of Aquino, an area his family held land in until 1137. He was born in Roccasecca, Italy.

He was the foremost classical proponent of natural theology, and the father of Thomism. His influence on Western thought is considerable, and much of modern philosophy was conceived in development or opposition of his ideas, particularly in the areas of ethics, natural law, metaphysics, and political theory. Unlike many currents in the Church of the time, Thomas embraced several ideas put forward by Aristotle — whom he referred to as "the Philosopher" — and attempted to synthethise Aristotelian philosophy with the principles of Christianity. The works for which he is best known are the Summa Theologica and the Summa contra Gentiles. His commentaries on Sacred Scripture and on Aristotle are an important part of his body of work. Furthermore, Thomas is distinguished for his eucharistic hymns, which form a part of the Church's liturgy.

Thomas is honored as a saint by the Catholic Church and is held to be the model teacher for those studying for the priesthood, and indeed the highest expression of both natural reason and speculative theology. In modern times, under papal directives, the study of his works was long used as a core of the required program of study for those seeking ordination as priests or deacons, as well as for those in religious formation and for other students of the sacred disciplines (Catholic philosophy, theology, history, liturgy, and canon law).

Also honored as a Doctor of the Church, Thomas is considered the Church's greatest theologian and philosopher. Pope Benedict XV declared: "This (Dominican) Order ... acquired new luster when the Church declared the teaching of Thomas to be her own and that Doctor, honored with the special praises of the Pontiffs, the master and patron of Catholic school