So why do I believe in the unerring fandango dance of the DNA, which I cannot fully imagine no matter how hard I try, and not fairies, which any child can imagine? To answer this question adequately would take a book, but to provide a brief answer I'll borrow a diagrammatic scheme proposed by physicist-philosopher Henry Margeneau.

Down the middle of a blank page Margeneau draws a vertical line that he calls the "perception plane." It represents the locus of our immediate sensations of the world -- sights, tastes, odors, touches, sounds -- the interface between the world as it is and the world as we know it. To the left of the line is the world "out there," which we know only through the windows of our senses. To the right of the line Margeneau draws circles representing "constructs" -- names, descriptions, or ideas we invent to make sense of our perceptions. The more abstract the construct, the farther he places the circle from the line.

Immediately adjacent to the perception plane are those constructs that correspond to direct sensations: "blue," "bitter," "pungent," "brittle," "shrill." The construct "dragonfly" is a bit further from the perception plane, but not very far away. I feel a sensation on my finger ("tingle"), I see a color ("blue"), a quality of light ("iridescent"), a shape ("long and narrow"), and I name this ensemble of sensations "dragonfly." Perhaps I am the first person to see a dragonfly, in which case I invent the name; I have added a circle to my conceptual map of the world ("dragonfly"), linked by short sturdy lines to other circles more closely adjacent to the perception plane ("tingle," "iridescent blue", "long and narrow"). Or perhaps I recognize a congruence between the current ensemble of sensations and other sensations stored in memory; I have seen a dragonfly before, or a picture of a dragonfly. In which case, the remembered construct is reinforced by my new experience; the links between the circles are made bolder, darker; the construct is anchored yet more firmly to the world "out there." I close my eyes and open them again; the links hold. I move my finger; the sensations change; the form flutters, moves away, I hear a whirr, the tingle ceases.

Perception and cognition are hugely complex processes, endlessly debated by psychologists, neurologists and philosophers. Margeneau's simple schematic of connected circles is itself only a construct, a useful way of describing the devilishly complex business of perception and cognition. The important thing is to realize that our ideas about the world are not the same as the world itself (a point often missed by True Believers). Nevertheless, only the most obtuse idealist would hesitate to call "dragonflies" real. In the case of dragonflies, at least, we are confident that the construct captures the reality.

What about the construct "fairy"? We should also put "fairy" rather close to the perception plane. After all, there is nothing terribly abstract about fairies; a fairy is a little person with dragonfly wings and there is nothing unfamiliar about any of that. Perhaps our ancestors invented the construct "fairy" because of some primitive intuition that we are not alone. Whatever the reason, once we have the construct, it is easy enough to make connections. A tool goes missing from the garden, or we hear a strange singing noise on the hill at night. Between the construct "fairy" and "missing tool" and "singing noise" we draw lines, anchoring "fairy" into our map of reality. What is missing from our map are lines connecting the construct "fairy" directly to immediate sensations. No one has actually seen a fairy.

"DNA replication" is a construct far removed from the perception plane. There is almost nothing about the construct that relates to ordinary experience, which is why the construct is so difficult to imagine. The perceptions upon which the construct is based are highly technical; for example, x-ray diffraction photographs and demanding chemical assays. The construct "double helix DNA" is connected with reality by way of many other technical constructs, circles connected to circles in a vast web, by as many paths as we can devise and test, until at last we reach the relevant immediate perceptions -- blackened grains in a photographic emulsion, for example, or a reading on a microbalance in the chemical lab -- perceptions that mean nothing except in the context of the entire web of constructs. The scientist looks for taut and unambiguous connections between constructs and perceptions that can be subjected to quantitative and reproducible experimental tests.

You will have noticed that I have used familiar metaphorical language in describing the activity of DNA: "fandango dance," "stairs," "bannisters," "treads," "zipper" and so on. It should be clear, however, that this language in itself does not anchor the construct "double helix DNA" firmly to reality. We do not suppose that the connection between a DNA molecule and a zipper is anything more than metaphorical. By contrast, to speak of a fairy as a "little person with dragonfly wings" is the essence of the construct.

If we accept Margeneau's schematic, and admit that "fairy" and "double helix DNA" are both mental constructs removed from immediate sensation, then why are we allowed to say that one construct is more or less "real" than the other? Should we consider "fairy" to be more real since it is closer to the perception plane and therefore more familiar? The True Believer has heard the singing on the hill. She has experienced the tool gone missing from the garden. Therefore fairies exist and that's the end of it. The Skeptic would say, however, that the links connecting "fairy" to the noise on the hill and to the missing tool are flimsy. The noise might be just as well be explained by the construct "wind," with links that are firmer, more reproducible, and more widely acknowledged. The missing tool might be attributed to absent-mindedness or human theft, both of which are universally acknowledged parts of our common experience. In other words, "fairy" is connected to immediate sensation by few and arbitrary lines. Snip away the construct "fairy" and the rest of the map stands firm, no sensations go unexplained.

Our understanding of DNA replication, on the other hand, is embedded in a vast and resonant web of interconnected constructs. It is the essence of scientific skepticism to test and re-test each link in the web, to try to prove it faulty, to look for more concise patterns of constructs and connections that will adequately explain our immediate sensations -- the blackened grains in the photographic emulsion, the results of the chemical assay. If we have succeeded in constructing a resonant web of constructs, then any observer, Skeptic or True Believer, should be able to trace the links back to the perceptional source along vibrant lines of connection. It is the firmness of these many connections, based upon tens of thousands of exact, quantitative, reproducible experiments that anchors the construct "double helix DNA" to reality. Snip a line of connection here and there -- the web still holds. Remove the construct entirely and sensations go unexplained. And that's why we believe in the seemingly impossible dance of the DNA.

But it isn't easy. Many of the links in the scientist's map of the world are highly technical. Only narrow specialists will comprehend some of the connections. Any one scientist must trust the veracity of all other scientists, which is why so much effort goes into quantitative data keeping, citation of relevant prior research, and peer review. A scientist giving a talk to fellow scientists, even to close colleagues, is unlikely to get very far before someone interrupts with "Now wait a minute, about that last step..." I have often watched the skeptical engine of science at work -- winnowing, pruning, testing the resilience of the web. You don't want to be on the receiving end of this kind of collective scrutiny unless your ducks are well in line.

Certainly it is easier to believe in fairies than in DNA. It is also more consoling, more self-edifying, more entertaining. Fairies play into the whole gamut of human emotions: love, fear, power, powerlessness, the "land of lost content" of childhood. But fairies are a concept we can do without and still make perfect sense of the world. We cannot do without the concept DNA -- and neither can True Believers, whether they know it or not.