How does the Church define reason? How does the Church define reason?
Does the Catechism of the Catholic Church give a definition? Fides et Ratio did not have a definition even if it is a encyclican on the subject.

I find this a bit strange.
It is not for the Catholic Church to define reason, but to teach how reason contributes or hinders our understanding of God, who is critical for our salvation:


When a philosophy (or science, which is properly below philosophy) devalues reason too much below what the Scripture or Tradition says, the church will re-establish the existence and competence of human reason.  For example, in light of Neural networks and Artificial Intelligence, some scientism position sees human being as nothing more than an animal with an advanced brain that one day can be completely simulated by a machine.
On the other hand, when Nietzsche dares people to "grow up" and make their own ethics and values, the Church will step in and say that reason has overstepped its authority because reason cannot supplant the natural law implanted in human conscience by God Himself.


Philosophy is free to define what reason and rationality is as long as certain aspects of human nature as taught by the Church stay intact.

Therefore Fides et Ratio's main purpose is to guide Catholics in the 20th century on how to make use or guard against modern philosophies in connection with applying the church doctrines in a believer's faith life:


It is a kind of re-establishing the boundary between the church's domain (charged with guarding the proper use of the revelation from God in Scripture and Tradition) and the philosophy's domain (charged with exploring everything that rationality can discover on its own)
It is a guide to reconcile the findings of both domain, because at the end of the day we cannot have parallel but contradictory truths that destroy each other's credibility.  There is only a single rationality and a single revelation to resolve a single matter.  Faith and Rationality must cooperate harmoniously to determine which sphere has the authority and if there is a conflict, which one takes precedence depending on the matter and the discipline.


Please see an excellent commentary on Fides et Ratio by Prof. Alfred J. Freddoso, a Catholic philosophy professor at Notre Dame who teaches courses on St. Thomas Aquinas as well.