Lewis’ second favorite proof, the argument from reason, appears in his book Miracles. A certain kind of naturalism, he observes, characterizes rational thinking as a mere product of nervous reflexes, instincts, and habits. Lewis replies that physical or psychological conditioning cannot explain our power to make judgments about truth and error. We are conscious that our judgments are determined not by subrational forces but by reality as it impinges on our minds. The power to reach understanding through rational explanations is evidence of an affinity between the mind and reality. It is explicable only if there is an aboriginal mind that accounts for both intelligence and intelligibility.
Lewis’ sketchy presentation of this argument leaves further work to be done. Having an ancestry that goes all the way back through Plato to Anaxagoras, it resembles the argument for the existence of God proposed in highly technical terms by Bernard Lonergan and popularized in several apologetical works of Hugo Meynell. For all these authors the wonderful correspondence between reason and reality implies that reality is imbued with an order that stems from a creative Mind. Lewis’ focus is not so much on the intelligibility of the world as on the mind’s capacity for truth, which in his opinion cannot be explained by natural selection but only by an intelligent Creator.
Lonergan's AFR is found in chapter 19 of his book Insight. Hugo Meynell's book The Intelligible Universe is based on Lonergan's argument.