C.S. Lewis described this book as a "preliminary study" on miracles. This description is fitting because he deals not with the question of whether miracles have actually happened, but with the prior question of whether they are possible. He begins by arguing against naturalism, a worldview that excludes the possibility of miraculous events. He then turns his attention to the probability of miracles, and, finally, to the nature and uniqueness of the Christian miracles. He does not try to prove Christianity. Rather, his aim is to remove impediments to clear thinking on the question of miracles that would prevent one from giving the Christian claims a fair hearing.

When we go to investigate claims about miracles, our philosophy (or worldview) will determine which explanations we're willing to consider. A naturalist, for example, has already decided there is nothing beyond nature, so no matter how extraordinary or unnatural a given event may seem, he will only entertain naturalistic explanations. Thus the question of whether miracles have actually occurred must be put on hold until we've answered the more fundamental philosophical question of whether miracles are even possible. To just assume the impossibility of miracles from the outset would be to beg the question.

A miracle is supernatural interference with nature; it is an event that nature could never produce on her own. If naturalists are right, and nature is all that exists, then miracles are obviously impossible. There can be no miraculous events if there is nothing beyond nature to cause them. Supernaturalists, on the other hand, believe that something else exists besides nature. They believe there’s more to reality than the natural, physical world. If they are right, then it is possible nature could be interfered with from beyond. That is, miracles could possibly occur. Thus whether miracles are possible depends on whether naturalism is true or false.

The naturalist, having decided there is nothing beyond the natural world, is committed to the view that nature is a self-contained, closed system. In other words, whatever happens within the system can be fully explained without having to look beyond it for answers. Since the beginning, the entire contents of the universe—from the largest galaxies to the smallest particles—have mechanistically obeyed the laws of physics and chemistry. It follows that each and every event is necessarily linked to prior events in a great causal chain that stretches all the way back to the beginning of the universe. There can be no exceptions. If naturalism is true, mindless, mechanistic causes are behind every event in history.

Against this background we can begin to see that naturalism faces an insurmountable difficulty. If all events—including mental events—are ultimately the result of underlying physical, mechanistic causes, then rational thought is excluded from reality. After all, a belief is rational only if it is supported by reasons and formed on that basis. In other words, to be rational, a belief must be seen to follow from, as a logical consequence, the grounds which support it. This logical insight must be the true cause of the belief's formation. If, instead, the cause of a belief is fundamentally physical—due, for example, to neurochemical changes in the brain—then logical grounds are irrelevant; the belief will form inevitably, whether it is grounded or not.

If you were to ask a naturalist why he believes naturalism is true, he would, if he were honest, have to say, “Because a series of interconnected physical causes and effects that stretches back to the beginning of the universe eventually resulted in the current state of physical affairs—and in this current state of physical affairs, my brain chemistry is such that I find myself affirming naturalism as true.” For any belief that forms in a human mind, this is the only explanation naturalism allows. By leaving logical grounds and rational insight out of the picture—or at the very least reducing them to the point of irrelevance—naturalism discredits every belief, including the belief that naturalism is true.

The supernaturalist avoids the naturalist’s difficulty because he is not committed to viewing acts of reasoning as purely natural events. His explanatory resources are not limited to impersonal forces and mindless mechanisms. He is therefore able and willing to say that acts of reasoning are unique, in the sense that they are independent from the interlocked, cause-and-effect system we call "nature." They involve and, indeed, require a special kind of causation that naturalism cannot allow. Just as the knowledge of how a machine works is not one of the machine's parts, so reason—that power which gives us knowledge of nature—is not one of nature's parts. There is something beyond nature at play whenever rational thoughts are formed.

If reason cannot come from non-reason (i.e., mindless, mechanistic nature), it must come, instead, from a transcendent rational source. The best explanation is that our minds bring reason into nature because they stem from an eternal, self-existent, rational being: God.

Reason is fundamental. We cannot give it up just because it doesn’t fit within a naturalistic framework. We must give up naturalism instead.

The same problem that applies to reason applies to moral judgments. We all believe there are things we ought and ought not do, but these beliefs are merely illusions if they have non-rational causes. Naturalists explain morality in terms of evolution—there is survival value to moral thought and behavior, so the feeling that some things are "right" and others "wrong" has been ingrained in us. But explaining how we came to believe in right and wrong is not the same as showing that right and wrong are real. On naturalism, moral judgments are just personal expressions of how mindless nature has conditioned us to feel. While such judgments may have survival value, there is no reason to think they also have truth value.

If we're to go on making moral judgments, then we have to give up this idea that moral beliefs are products of mindless nature. Like reason, the validity of human conscience depends on its being rooted in a source independent from nature.

Some object to the supernatural because they think it should be more obvious. They seem to have a point. However, some things are so obvious they escape our notice. We can read for hours, for example, without ever thinking about our eyes. We have no awareness of them not because they're obscure or remote, but because they're such an intimate part of what we're doing. The supernatural can go unnoticed for the same reason. Naturalists are too busy thinking about nature to consider the implications of thought itself. But once we've thought long and hard about thought—specifically, about what reality must be like in order for our thinking to be valid—the supernatural implications of reason become readily apparent. 

Another popular objection is based on advances in modern science. These advances, we’re told, have revealed that nature operates according to strict laws, making it impossible for us to believe in miracles. But why should nature's laws exclude miracles? A miracle, by definition, is an exception. There can be no exceptions in a world without rules or regularity. Far from being excluded by nature's laws, miracles only make sense in a lawful universe; they would be meaningless and imperceptible in any other. The important question is not whether nature obeys laws—we all agree that it does—but whether those laws can be suspended. The mere discovery of laws in nature does not even begin to answer that question.

Is there anything about nature that would make her impervious to miracles, even if God exists? It is widely believed that the laws of nature are necessary truths, like the truths of mathematics. In that case, a miracle—when conceived of as a violation of nature's laws—would be a contradiction in terms, like a married bachelor or a round square. In other words, it would be an impossibility, even for God.

But it is a mistake to think of miracles as violations of nature's laws. If you put some spare change in a drawer, and find that some of it is missing the next day, a criminal statute may have been violated, but the laws of arithmetic have not been violated; they will always tell you how much money you have in your drawer, whether a thief has stolen some or not. This illustration shows that the laws of arithmetic will help you identify theft, but they will not prevent its occurrence. In the same way, nature's laws will help you identify a miracle, but they will not prevent it, and they will not be violated by it. The outside interference—or tampering, if you will—merely injects a new event into the equation and the laws take over from there. They accommodate; they do not exclude.

If God works miracles, He does so like the thief. His activity will not be prohibited by nature’s laws; rather, it will be exposed by them.

Many are content to allow for supernaturalism in some form, just not the form taken by Christianity where miracles are a central theme. In their view, the Christian story is loaded with primitive ideas and imagery. They scoff at the idea of a God—pictured as an old man—who sits on a throne up in a sky palace somewhere and once bore a son with super-human abilities. It all seems rather childish and silly.

Lewis admits he once thought just like these impatient skeptics. But he eventually learned that we must distinguish between the imagery associated with the Christian doctrines—which can be rather primitive—and their core, underlying meaning. This is because mental pictures can be false or inaccurate, even though the thinking associated with them is perfectly sound. For example, a young child might come to believe—mistakenly—that poison is always red. But this false belief does not invalidate his entire concept of poison. He may still believe—correctly—that poison can be deadly. 

With this consideration in mind, we can return to the Christian doctrines and see that just because there are crude mental pictures associated with them does not mean the doctrines themselves are false, or that you must be a fool to believe them. To some extent, inadequate mental pictures accompany all of our beliefs. If we write off Christianity on their account, we must write off everything else, too. 

Some maintain that it would be beneath God to work miracles. The idea of His tinkering with nature suggests that He didn't get things right the first time. If we’re to think of God as competent, we must deny that He works miracles. This view is understandable, but it stems from an error.

When we learn to write as children, we're taught to follow strict grammatical rules. But many great writers—think Virgil or Shakespeare—seem perfectly willing to break the rules and do things our teachers said we shouldn't. In doing so, they are not making mistakes; rather, they are skilled to the point where they can take certain liberties. Seeing a bigger picture than we see—and abounding in talent that we lack—they are able to elevate their work above the baseline standard that applies to the rest of us. Their commitment to the rules is appropriately relaxed, while ours, on account of our limitations, is advisedly rigid.

The people who think it beneath God for Him to work miracles are like the students who criticize a great writer’s artistic license. If they only knew more, and saw the bigger picture, their criticism might turn to praise. 

David Hume's take on miracles has become predominant. According to Hume, the probability of miracles hinges on past experience, and since we always—without exception—experience nature behaving regularly, we should always consider miracles more improbable than anything else. If we have to choose between believing a witness to a miracle and believing in the regularity of nature, past experience dictates that we should believe in the regularity of nature.

The problem with this view is its vicious circularity. By saying that miracles have never occurred because we have uniform experience of nature’s regularity, Hume is really saying, “Miracles have never occurred because miracles have never occurred.” This gets us nowhere. We need a more sensible standard for judging miracle reports.

To say that God took on human form is to say that the supernatural has merged with the natural. This might be too incredible to believe if we did not already have first-hand experience of something similar. As we saw earlier, we are not merely natural creatures. Whenever we reason, we are conjoining nature (our brains) with supernature (our power of rationality). Our experience as rational agents makes it easier to see how God’s spirit could plausibly enter nature to join with a man's body. While such a union cannot be fully comprehended, neither can it be denied on the basis that we've never seen anything like it. Something like it—a shadowy reflection of it—occurs whenever we reason. In this sense, the miracle of the Incarnation fits with what we already know.

We see reflections of the Incarnation in nature, as well. Plant life, for example, goes through a cycle of death and rebirth, descent and reascent. Seeds fall to the ground and rise up again in new life. Similarly, God came down to earth and reascended; He died and was buried, only to rise again and bring the world back up with Him. 

This central Christian miracle—if it happened—harmonizes and unifies. Rather than creating difficulties, it resolves them. Miracles of this sort do not necessarily require extraordinary evidence, as they are already right at home with what we know.

When we read fairy tales and mythology, we encounter bizarre, seemingly random miracles. They are the kind of miracles you would expect if nature was being interfered with not by her own Creator King, but by some foreign invader. If they really happened, they would create more difficulties than they resolved.

The Christian miracles are strikingly different. If there is, in fact, a Creator King who made nature in the first place, and if He were to interfere with His creation and work miracles, we would expect Him to act like Christianity says He has. There is a fitness to the Christian miracles that increases their credibility and sets them apart.

When Jesus turned water into wine, for example, He was doing nothing new. God, after all, is the creator of grapes and water, soil and sunlight. He is the great chemist who invented fermentation. In a sense, He is always turning water into wine. In one particular instance, at the wedding in Cana, the incarnate God simply bypassed the slower, natural process. While the speed of the transformation was different, the input (water) and output (wine) were the same. This miracle, among many others, demonstrated that Christ was the God of nature.

When you’re ready to study the historical evidence for the Christian miracles, start with the original text of the New Testament first, then proceed cautiously to modern commentaries. The word “cautiously” cannot be stressed enough. Naturalistic assumptions and fallacies will greet you at every turn. You must be vigilant, guarding your mind against them. Be on the lookout for sleight of hand in the arguments—whether subconscious or deliberate—that serves to smuggle in unwarranted assumptions or unreasonable doubt.

When you return to your daily routine, it will be easy to slip back into your old mode of thinking. We are naturally and habitually resistant to certain truths. You may find yourself, in certain moods or moments, inclined to think miracles could never happen. A feeling that the world must not be like that may come over you. This inclination toward naturalism can be strong. You must constantly remind yourself that such feelings disprove nothing. Miracles are either possible or not, wholly apart from our feelings.