The Theo Faron we meet at the beginning of P.D. James' The Children of Men is not the same man we see by the end of the story. He has changed greatly from the detached, isolated, somewhat cynical man he was to a person willing to risk everything to do what he feels is right. Along the way, he discovers beauty and love. He learns that there are more things in life than power or the desire to keep his nose out of everyone else's business.
Theo does not make this change by either his emotions or his intelligence. Rather, he changes by means of both his emotions and his intelligence. They are both operative in his wake-up call and his growth. When Theo sees Julian, for instance, he feels something of an attraction toward her, and he agrees to meet with the group of dissidents to which she belongs. At this point their intellectual arguments fail to convince him, yet he continues to associate with them, drawn perhaps by an emotional force he does not fully understand.
When he witnesses the Quietus, Theo is horrified by the brutal treatment of the elderly victims. He even rushes forward to try to save Hilda. His emotions are in full play here, yet he is beginning to realize intellectually that the government has long since been degrading, even torturing, its citizens. He uses intellectual arguments to try to convince Xan, but he fails.
When Theo returns from his escape to Europe, he again is driven by both emotions and intelligence. He finds Jasper dead and is horrified, but he also understands the reason for that death. As he flees with the Five Fishes, Theo cares for Julian and even helps her deliver her child, showing significant love for her. When the child is born, Theo even feels a deep joy. But he also continues to think, planning and trying to anticipate Xan's moves, and he is ready for Xan when his cousin finally appears.
Indeed, emotions and intellect have both guided Theo to becoming a better person, a person who has discovered love and beauty in a world filled with hatred and ugliness.