Look around the room you're sitting in. Pick out four items at random. I'm doing it now and my items are a desk, a television, a door and a pencil. Now, make the words you have chosen into a sentence using as few additional words as possible. For example: "I was sitting at my desk, looking at the television, when a pencil fell off and rolled to the door." Or: "The television close to the door obscured my view of the desk and the pencil I needed." Or: "The pencil on my desk was pointed towards the door and away from the television." You will find that you can always do this exercise - and you could do it for ever.

That's the easy part. The hard part is to answer this question: what did you just do? How were you able to turn a random list into a sentence? It might take you a little while but, in time, you will figure it out and say something like this: "I put the relationships in." That is to say, you arranged the words so that they were linked up to the others by relationships of cause, effect, contiguity, similarity, subordination, place, manner and so on (but not too far on; the relationships are finite). Once you have managed this - and you do it all the time in speech, effortlessly and unselfconsciously - hitherto discrete items participate in the making of a little world in which actors, actions and the objects of actions interact in ways that are precisely represented.

This little miracle you have performed is called a sentence and we are now in a position to define it: a sentence is a structure of logical relationships. Notice how different this is from the usual definitions such as, "A sentence is built out of the eight parts of speech," or, “A sentence is an independent clause containing a subject and a predicate," or, "A sentence is a complete thought." These definitions seem like declarations out of a fog that they deepen. The words are offered as if they explained everything, but each demands an explanation.

When you know that a sentence is a structure of logical relationships, you know two things: what a sentence is - what must be achieved for there to be focused thought and communication - and when a sentence that you are trying to write goes wrong. This happens when the relationships that allow sense to be sharpened are missing or when there are too many of them for comfort (a goal in writing poetry but a problem in writing sentences). In such cases, the components of what you aspired to make into a sentence stand alone, isolated; they hang out there in space and turn back into items on a list.

Armed with this knowledge, you can begin to look at your own sentences and those of others with a view to discerning what is successful and unsuccessful about them. As you do this, you will be deepening your understanding of what a sentence is and introducing yourself to the myriad ways in which logical structures of verbal thought can be built, unbuilt, elaborated upon and admired.

My new book, How to Write a Sentence, is a light-hearted manual of instruction designed to teach you how to do these things - how to write a sentence and how to appreciate in analytical detail the sentences produced by authors who knock your socks off. These two aspects - lessons in sentence craft and lessons in sentence appreciation - reinforce each other; the better able you are to appreciate great sentences, the closer you are to being able to write one. An intimate knowledge of what makes sentences work is one prerequisite for writing them.

Consider the first of those aspects - sentence craft. The chief lesson here is: "It's not the thought that counts." By that, I mean that skill in writing sentences is a matter of understanding and mastering form not content. The usual commonplace wisdom is that you have to write about something, but actually you don't. The exercise I introduced above would work even if your list was made up of nonsense words, as long as each word came tagged with its formal identification - actor, action, object of action, modifier, conjunction, and so on. You could still tie those nonsense words together in ligatures of relationships and come up with perfectly formed sentences like Noam Chomsky's "Colourless green ideas sleep furiously," or the stanzas of Lewis Carroll's "Jabberwocky".

If what you want to do is become facile (in a good sense) in producing sentences, the sentences with which you practise should be as banal and substantively inconsequential as possible; for then you will not be tempted to be interested in them. The moment that interest comes to the fore, the focus on craft will be lost. (I know that this sounds counter-intuitive, but stick with me.)

I call this the Karate Kid method of learning to write. In that 1984 cult movie (recently remade), the title figure learns how to fight not by participating in a match but by repeating (endlessly and pointlessly, it seems to him) the purely formal motions of waxing cars and painting fences. The idea is that when you are ready either to compete or to say something that is meaningful and means something to you, the forms you have mastered and internalised will generate the content that would have remained inchoate (at best) without them.

These points can be illustrated with senten­ces that are too good to be tossed aside. In the book, I use them to make points about form, but I can't resist their power or the desire to explain it. When that happens, content returns to my exposition and I shift into full appreciation mode, caressing these extraordinary verbal productions even as I analyse them. I become like a sports commentator, crying, "Did you see that?" or "How could he have pulled that off?" or "How could she keep it going so long and still not lose us?" In the end, the apostle of form surrenders to substance, or rather, to the pleasure of seeing substance emerge though the brilliant deployment of forms.

As a counterpoint to that brilliance, let me hazard an imitation of two of the marvels I discuss. Take Swift's sublimely malign sentence, "Last week I saw a woman flayed and you will hardly believe how much it altered her person for the worse." And then consider this decidedly lame imitation: "Last night I ate six whole pizzas and you would hardly believe how sick I was."

Or compare John Updike's description in the New Yorker of the home run that the baseball player Ted Williams hit on his last at-bat in 1960 - "It was in the books while it was still in the sky" - to "He had won the match before the first serve." My efforts in this vein are lessons both in form and humility.

The two strands of my argument can be brought together by considering sentences that are about their own form and unfolding; sentences that meditate on or burst their own limitations, and thus remind us of why we have to write sentences in the first place - we are mortal and finite - and of what rewards may await us in a realm where sentences need no longer be fashioned. Here is such a sentence by the metaphysical poet John Donne:

If we consider eternity, into that time never entered; eternity is not an everlasting flux of time, but time is a short parenthesis in a long period; and eternity had been the same as it is, though time had never been.

The content of the sentence is the unreality of time in the context of eternity, but because a sentence is necessarily a temporal thing, it undermines that insight by being. (Asserting in time the unreality of time won't do the trick.) Donne does his best to undermine the undermining by making the sentence a reflection on its fatal finitude. No matter how long it is, no matter what its pretension to a finality of statement, it will be a short parenthesis in an enunciation without beginning, middle or end. That enunciation alone is in possession of the present - "is" - and what the sentence comes to rest on is the declaration of its already having passed into the state of non-existence: "had never been".

Donne's sentence is in my book; my analysis of it is not. I am grateful to the New Statesman for the opportunity to produce it and to demonstrate once again the critic's inadequacy to his object.

Stanley Fish is Davidson-Kahn Distinguished University Professor of Humanities and Law at Florida International University. His latest book is "How to Write a Sentence: and How to Read One" (HarperCollins, £12.99)