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.
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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:
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:
How does the Church define reason?
Catholic Culture gives us a short, but easy definition as to what reason is.
In general, the mind in its function of attaining the truth. Also the basis or evidence used by the mind in its pursuit of truth. It differs from the intellect, whose proper role is to perceive the truth, whether arrived at by a reasoning process or perceived immediately as intuition. Reason, therefore, is a process, where intellect is possession.
The Catholic Encyclopædia gives a much more detailed description as to what reason is.
The closest I can find in the Catechism of the Catholic Church is as follows:
286 Human intelligence is surely already capable of finding a response to the question of origins. The existence of God the Creator can be known with certainty through his works, by the light of human reason, even if this knowledge is often obscured and disfigured by error. This is why faith comes to confirm and enlighten reason in the correct understanding of this truth: "By faith we understand that the world was created by the word of God, so that what is seen was made out of things which do not appear."
Chapter 4 "On Faith and Reason" of the First Vatican Council's Dei Filius describes human reason by contrasting it with knowledge obtained by faith from divine Revelation:
The Catholic Church, with one consent, has also ever held and does hold that there is a twofold order of knowledge distinct both in principle and also in object; in principle, because our knowledge in the one is by natural reason, and in the other by divine faith; in object, because, besides those things to which natural reason can attain, there are proposed to our belief mysteries hidden in God, which, unless divinely revealed, can not be known.