Summa contra Gentiles

A treatise in four books, written between about 1259 and 1265, also called the Book on the Truth of the Catholic Faith against the Errors of Unbelievers. An old story says it was written for Dominican missionaries in Spain who dealt with Muslims and Jews, but its real audience is debated. Since non-Christians do not accept the Bible, Aquinas proceeds mainly by natural reason in the first three books. Book I proves that God exists, using Aristotle's argument from motion, then derives God's attributes, such as simplicity, perfection, goodness, knowledge and will. Book II treats creation, argues that reason can neither prove nor disprove that the world had a beginning, and defends the soul as the form of the body against the view that all humans share one intellect. Book III argues that human happiness lies in the contemplation of God, and discusses evil, providence, law and grace. Book IV turns to truths known only by revelation, such as the Trinity, the Incarnation and the resurrection.

It is Aquinas's fullest attempt to show how far reason alone can go on God, the soul and the good life, and a clear guide to where, in his view, faith must take over.