Many Christian apologists have assimilated Lewis to their own way of thinking, presenting him in thoroughly modernist terms as an advocate of rationalist defences of faith. Yet to get the most out of reading Lewis, we need to approach him on his own terms. Here, I want to explore Lewis's distinctive understanding of the rationality of faith, which emphasises the reasonableness of Christianity without imprisoning it within an impersonal and austere rationalism.

I came to appreciate this distinctive approach when researching my recent biography of Lewis. For reasons I do not understand, the importance of Lewis's extensive use of visual images as metaphors of truth has been largely overlooked. For Lewis, truth is about seeing things rightly, grasping their deep interconnection. Truth is something that we see, rather than something we express primarily in logical or conceptual terms.

The basic idea is found in Dante's Paradiso (XXIII, 55-6), where the great Florentine poet and theologian expresses the idea that Christianity provides a vision of things - something wonderful that can be seen, yet proves resistant to verbal expression:

From that moment onwards my power of sight exceeded

That of speech, which fails at such a vision.

Hints of such an approach are also found in the writings of G.K. Chesterton, whom Lewis admired considerably. For Chesterton, a good theory allows us to see things properly: "We put on the theory, like a magic hat, and history becomes translucent like a house of glass." Thus, for Chesterton, a good theory is to be judged by the amount of illumination it offers, and its capacity to accommodate what we see in the world around us and experience within us: "With this idea once inside our heads, a million things become transparent as if a lamp were lit behind them." In the same way, Chesterton argued, Christianity validates itself by its ability to make sense of our observations of the world: "The phenomenon does not prove religion, but religion explains the phenomenon."

For Lewis, the Christian faith offers us a means of seeing things properly - as they really are, despite their outward appearances. Christianity provides an intellectually capacious and imaginatively satisfying way of seeing things, and grasping their interconnectedness, even if we find it difficult to express this in words. Lewis's affirmation of the reasonableness of the Christian faith rests on his own quite distinct way of seeing the rationality of the created order, and its ultimate grounding in God. Using a powerful visual image, Lewis invites us to see God as both the ground of the rationality of the world, and the one who enables us to grasp that rationality: "I believe in Christianity as I believe that the Sun has risen, not only because I see it but because by it I see everything else." Lewis invites us to see Christianity as offering us a standpoint from which we may survey things, and grasp their intrinsic coherence. We see how things connect together.

Lewis consistently uses a remarkably wide range of visual metaphors - such as sun, light, blindness and shadows - to help us understand the nature of a true understanding of things. This has two important outcomes. First, it means that Lewis sees reason and imagination as existing in a collaborative, not competitive, relationship. Second, it leads Lewis to make extensive use of analogies in his apologetics, to enable us to see things in a new way.

For example, Lewis's famous apologetic for the doctrine of the Trinity in Mere Christianity suggests that our difficulties with this notion arise primarily because we fail to see it properly. If we see it another way - as, for instance, an inhabitant of a two-dimensional world might try to grasp and describe the structure of a three-dimensional reality - then we begin to grasp its intrinsic rationality.

Lewis's apologetics thus often takes the form of a visual imperative: "Try seeing it this way!" Lewis rightly realized that many people frame their accounts of things analogically, using a process that Hilaire Belloc called parallelism: the "illustration of some unperceived truth by its exact consonance with the reflection of a truth already known and perceived." Lewis does not try to prove the existence of God on a priori grounds. Rather, Lewis invites us to see how what we observe in the world around us and experience within us fits the Christian way of seeing things. Lewis's genius as an apologist lay in his ability to show how a Christian "viewpoint" (or, to borrow a term from Plato, a synoptikon) was able to offer a more satisfactory explanation of common human experience than its rivals - especially the atheism he himself had once espoused.

Throughout his apologetic writings, such as Mere Christianity, Lewis appeals to shared human experience and observation. How do we make sense of what we experience within us, or observe outside us? Lewis's apologetic approach is thus to demonstrate how an observation or experience fits, naturally and plausibly, within a Christian way of looking at things. Take his "argument from desire." This is not really an argument at all. It is more about observing and affirming the fit between a theory and observation. It is like trying on a hat or shirt for size, and looking at yourself in a mirror. How well does it fit? How many of our observations of the world can a theory accommodate, and how persuasively? It is basically about seeing how our experiences of desire fit a Christian framework.

Lewis thus argues that we experience desires that no experience in this world seems able to satisfy. And when we see these experiences through the lens of the Christian faith, we realize that this sort of experience is exactly what we would expect. Christianity tells us that that this is not our true home, and that we were created for heaven. How does that framework help us see these experiences? For Lewis, the answer was clear: "If I find in myself a desire which no experience in this world can satisfy, the most probable explanation is that I was made for another world."

Lewis's appeal here is not so much to cold rationalism, as to intuition and imagination. It is not a deductive argument, but an imaginative dynamic of discovery. Lewis invites his audience to see their experiences through a set of Christian spectacles, and to notice how these bring what might otherwise seem to be fuzzy or blurred into sharp focus. A pattern is thereby seen for the first time. For Lewis, the ability of the Christian faith to accommodate such things, naturally and easily, is an indicator of its truth.

The same approach is found in Lewis's "argument from morality." This is sometimes portrayed in ridiculously simplistic terms - for example, "experiencing a sense of moral obligation proves there is a God." Lewis did not say this, and he certainly did not think this. As with the "argument from desire," his argument is rather that the common human experience of a sense of moral obligation is easily and naturally accommodated within a Christian framework. The Christian lens brings things into focus. It enlightens the landscape of reality, allowing us to see how God, desire and morality are all held together within a greater scheme of things.

Lewis helps us to appreciate that apologetics need not take the form of deductive argument. It can be presented as an invitation to step into the Christian way of seeing things, and explore how things look when seen from its standpoint: "Try seeing things this way!" If worldviews or metanarratives can be compared to lenses, which of them brings things into sharpest focus? This is not an irrational retreat from reason. Rather, it is about grasping a deeper order of things which is more easily accessed by the imagination than by reason. Yet once seen, its intrinsic rationality can be appreciated.

Lewis's explicit appeal to reason thus involves an implicit appeal to the imagination. Perhaps this helps us understand why Lewis appeals to both modern and postmodern people. Lewis gives us a synoptikon which bridges the great divide between modernity and postmodernity, insisting that each outlook has its strengths because it is part of a greater whole. Their weaknesses arise when they pretend to offer the full picture, when they really offer only part of the whole. Once the "big picture" is seen, they are both seen in their proper light.

Lewis enriches our vision of apologetics, allowing us to affirm that Christianity makes sense, without limiting it to the "glib and shallow" rationalism that he himself once knew as an atheist. Reason and imagination are woven together, using a rich concept of truth which emphasizes how we come to see things properly, and grasp their inner coherence. Truth, beauty and goodness all have their part to play in Lewis's apologetics.

Such an "imaginative apologetics" allows us to affirm the reasonableness of faith, while at the same time displaying its power to captivate the imagination. The Christian churches need to ensure that their preaching, witness and worship express this same rich vision of reality, and lead others to wonder how they can go "further up and further in" to the landscape of faith.

Alister McGrath is the Andreas Idreos Professor of Science and Religion at Oxford University. He is the author of two substantial studies of Lewis: C.S. Lewis - A Life: Eccentric Genius, Reluctant Prophet and The Intellectual World of C.S. Lewis.