The Summa Theologica is primarily concerned with the relationship between faith and reason. Aquinas argues that faith and reason are not in conflict, but rather that they complement each other.

Aquinas defines faith as 'the substance of things hoped for, the evidence of things not seen' (Hebrews 11:1). He argues that faith is a virtue that allows us to believe in things that we cannot see or prove.

Aquinas defines reason as 'the power of the soul by which we know things in their causes' (Summa Theologica, I, q. 79, a. 1). He argues that reason is a natural faculty that allows us to understand the world around us and to make moral judgments.

Aquinas argues that faith is based on revelation, while reason is based on natural knowledge. He argues that these two sources of knowledge are not in conflict, but rather that they complement each other.

The Summa Theologica's most famous argument for the existence of God is the cosmological argument. This argument begins with the observation that the universe is contingent, meaning that it could have not existed. Aquinas then argues that if the universe is contingent, then it must have been caused by something that is necessary, meaning that it could not not exist. This necessary being is God.

The Summa Theologica's most famous argument for the immortality of the soul is the teleological argument. This argument begins with the observation that the soul has a natural desire for happiness. Aquinas then argues that this desire can only be satisfied by something that is infinite and eternal. This infinite and eternal being is God. Therefore, the soul must be immortal in order to achieve its natural desire for happiness.

The Summa Theologica's most famous argument for the existence of free will is the moral argument. This argument begins with the observation that we are morally responsible for our actions. Aquinas then argues that if we are morally responsible for our actions, then we must have free will. Otherwise, we would not be able to control our actions and we would not be responsible for them.

The Summa Theologica's most famous argument for the existence of God's providence is the teleological argument. This argument begins with the observation that the universe is ordered and purposeful. Aquinas then argues that this order and purpose must have been designed by a intelligent being. This intelligent being is God. Therefore, God must exist and he must be provident.

The Summa Theologica's most famous argument for the existence of the Trinity is the ontological argument. This argument begins with the observation that God is the most perfect being. Aquinas then argues that if God is the most perfect being, then he must be three persons in one substance. Otherwise, he would not be the most perfect being.

The Summa Theologica's most famous argument for the existence of the sacraments is the teleological argument. This argument begins with the observation that the sacraments are necessary for our salvation. Aquinas then argues that if the sacraments are necessary for our salvation, then they must have been instituted by God. Otherwise, we would not be able to be saved.

The Summa Theologica's most famous argument for the existence of the Church is the teleological argument. This argument begins with the observation that the Church is necessary for our salvation. Aquinas then argues that if the Church is necessary for our salvation, then it must have been instituted by God. Otherwise, we would not be able to be saved.

The Summa Theologica's most famous argument for the existence of the papacy is the teleological argument. This argument begins with the observation that the papacy is necessary for the unity of the Church. Aquinas then argues that if the papacy is necessary for the unity of the Church, then it must have been instituted by God. Otherwise, the Church would not be able to be united.

The Summa Theologica's most famous argument for the existence of the Eucharist is the teleological argument. This argument begins with the observation that the Eucharist is necessary for our salvation. Aquinas then argues that if the Eucharist is necessary for our salvation, then it must have been instituted by God. Otherwise, we would not be able to be saved.

The Summa Theologica's most famous argument for the existence of the resurrection of the dead is the teleological argument. This argument begins with the observation that the resurrection of the dead is necessary for our salvation. Aquinas then argues that if the resurrection of the dead is necessary for our salvation, then it must have been promised by God. Otherwise, we would not be able to be saved.

The Summa Theologica's most famous argument for the existence of the final judgment is the moral argument. This argument begins with the observation that we are morally responsible for our actions. Aquinas then argues that if we are morally responsible for our actions, then we must be judged for them. Otherwise, there would be no justice in the world.