Philosophy is a work of human reason alone. Theology also has a partition that deals with human reason alone. The medievals called that philosophical theology, or natural theology. Such as Aquinas’ “Quinque Viae” or his five ways of proving the existence of God. In which no faith was used, just reason and logic. When Aquinas states that faith and reason are always in harmony, he does not mean the psychological acts of believing and reasoning, but the objective truths that can be known by them.

The first question that Aquinas asks in his Summa Theologiae is whether faith and reason can be in harmony. In short, his answer is that if reason proclaims truth, and faith proclaims truth, truth cannot contradict itself or else it ceases to be truth. Therefore, faith and reason can be in harmony.

His second argument states that God is the author of the universe and God is also the author of faith. And since a teacher cannot contradict himself (unless his teaching is fictitious, which it is improper to say of God), hence, we can say that if something about the universe contradicts something about faith, there is an error in one of the arguments. Since truth cannot contradict truth.

He also goes on further to say that every argument against any teaching of Catholicism has a rational mistake in it somewhere, and therefore can be answered by reason alone without appeal to faith. Aquinas is not claiming that every Catholic teaching can be proved by reason, only that none can be disproved.

“St. Bonaventura, Aquinas’s Franciscan friend and contemporary, complained that Aquinas’s use of Aristotle diluted the wine of the Gospel by the water of pagan philosophy. Aquinas replied, “No, I am transforming water into wine.” For Aquinas, all reason is faith’s ally because all truth is God’s truth.” (pp 13)