There has been in my acquaintance at least the occasional person who thinks that faith and reason are opposed by definition. That is, as if "faith" be properly defined as
(F) "the assertion of a proposition that contradicts what is known by reason".
The main problem with Sense F (where 'F' stands for 'foolish' or 'failed') is that nobody who has faith in anything agrees with that definition. On the basis of that definition, to attack someone who professes faith is to attack a straw man.
It is no good to bring up the example of the fundamentalist whose faith contradicts the assertion of some standard scientific model. No scientific theory can be proved true, and so its assertions (like the age of the Earth) that cannot be observed directly by the senses are not known to be true. No scientific theory--and no assertion of the reality of an unobservable aspect of a theory--is ever the conclusion of a deductively valid argument. No such thing is ever known by reason. At most, one can know through science what the best current theory is and what the history of sense perceptions recorded in the results of all the scientific experiments conducted so far is. But a fundamentalist's claim about the age of the Earth is not intrinsically opposed to anything that is actually known by way of science. Regardless of how silly a fundamentalist's claims might be, they do not necessarily contradict what is known by reason, even if they do contradict claims made by the best current theory. So Sense F of the word "faith" doesn't even apply where almost everyone might think it applies.
It is no good to bring up the example of the fundamentalist whose faith contradicts the assertion of some standard scientific model. No scientific theory can be proved true, and so its assertions (like the age of the Earth) that cannot be observed directly by the senses are not known to be true. No scientific theory--and no assertion of the reality of an unobservable aspect of a theory--is ever the conclusion of a deductively valid argument. No such thing is ever known by reason. At most, one can know through science what the best current theory is and what the history of sense perceptions recorded in the results of all the scientific experiments conducted so far is. But a fundamentalist's claim about the age of the Earth is not intrinsically opposed to anything that is actually known by way of science. Regardless of how silly a fundamentalist's claims might be, they do not necessarily contradict what is known by reason, even if they do contradict claims made by the best current theory. So Sense F of the word "faith" doesn't even apply where almost everyone might think it applies.
There are, however, at least two genuine and relevant senses in which one might use "faith":
(A) "the assertion of a proposition that neither is known immediately by the mind, nor is known by reason, nor contradicts what is known by reason" and
(B) "the conviction or courage to believe what reason concludes, regardless of how difficult or apparently unpleasant such a conclusion might be". (C. S. Lewis, I think, invoked this sense somewhere in his writings.)
In Sense B, faith and reason are clearly allies: Faith is the act of the will by which one submits to the conclusions of reason.
Let us focus for a moment, then, on Sense A. Let us see how subtly important faith is to the very act of reasoning.
Reason involves the comprehension and construction of a chain of syllogisms. The conclusion of any syllogism is known to be true if its premises be true. Tracing any chain of reasoning back far enough, one arrives eventually at premises that cannot be proved true. One arrives at premises, none of which is itself the conclusion of a syllogism. In order for anything to be known on the basis of reason, some initial premises must be known not on the basis of reason.
Scientific theory provides a useful example. Reason develops the theorems of ordinary geometry on the basis of Euclid's postulates. Are the conclusions of ordinary geometry true for the space that we experience? That's a question of physics, not of mathematics, because the question is whether a mathematical description of something physical is true. Notwithstanding the facts of everyday experience, the behavior of the GPS navigation system depends on the theory of general relativity, which denies at least one of Euclid's postulates, which historically had been taken as self-evident. Even in science, one would need faith in postulates of one or another theory in order to believe that the product of reason is a true assertion. I do not believe that the postulate of a scientific theory ought ever to be regarded as true, as an object of faith--after all, when a theory is ruled out by observation, it then becomes clear that at least one of the theory's postulates must be false--but it would be necessary to have faith in a scientific theory's postulates in order to know that it is true.
Perhaps there are a few postulates of geometry, of number theory, of set theory, of metaphysics, of morality, etc., that can be known directly by the mind, even if no scientific theory could ever be built solely on these. Still, the reasonable person who would obstinately deny the legitimacy even of faith as in Sense A is driven toward at least a very anemic epistemology and perhaps even toward nihilism.
Catholic doctrine stands opposed to epistemologic anemia. It is developed by way of reason. It is like mathematics in that each of a small number of fundamental assertions is held as true (in this case, by revelation), and logic is applied to develop conclusions from them. Unlike a fundamentalist form of Christianity, the Church does not insist on the Bible as the source of myriad clearly defined assertions, every one of which must be true. While she teaches that the Bible is inerrant, she is silent on the specific meaning of most of the scripture. Rather, she provides a relatively small number of fundamental assertions that can be known only on the basis of revelation, which includes aspects of the Apostolic Tradition that were not recorded in scripture. Logic applied to the fundamental assertions in the light of new circumstances through history allows for the development of doctrine in a way analogous to the development of a branch of mathematics. Unlike the mathematical community or any other community of rational thinkers, however, the Church makes newly definitive assertions guaranteed by the Holy Spirit to be true, not to contain any error in the logic that leads from the fundamental, revealed principles to the newly definitive assertions.
