Faith is represented by the character of Psyche and reason is represented by the character of the Fox, Lysias.  The result of the tension between the two is simply that reason can not sufficiently answer metaphysical questions. Reason can answer what it is, but not why it is.  Psyche, while educated by the Fox and well versed in the use of reason, is able to perceive the great castle given to her by the God of the Grey Mountain because she is willing to believe that things exist that cannot be perceived and tested by reason.  She has faith in the God of the Mountain.  Orual, Psyche's sister who was also educated by the Fox, mistrusts faith and tries to rely solely on reason.  Orual does this for the dual motive that reason is the method of her prized teacher and also because faith will deprive her of her treasured sister, Psyche, who wishes to remain with the God of the Mountain rather than return to the castle with Orual.  Using purely rational arguments along with a bit of emotional blackmail, Orual persuades Psyche to betray the God of the Mountain and test him by rational means rather than rely on him through faith.  Orual's failure here to obey the dictates of faith (she catches a glimpse of the God of the Mountain and has cause to have faith) causes Psyche to be rejected by the God of the Mountain and shows the weaknesses of purely rational pursuits without the mediating influence of faith.