And so to jail

MICHELLE TAYLOR, now beginning her third day in prison for the murder of Alison Shaughnessy, had written in her diary: 'My dream solution would be for Alison to disappear as if she never existed, and then maybe I could give everything to the man I love.'

Why did Michelle risk writing down such incriminating and hostile thoughts? Was it because the relief of confession was too great to resist; or because she believed strangers - let alone detectives - would never intrude upon her private diary? What, for that matter, makes anyone keep a diary?

There is a paradox here. The reason people confide so recklessly is that diaries are assumed to be secret, the reading of them a dreadful invasion of privacy. Yet a diary unseen by anyone except its author is powerless and even pointless. It may be a wonderful literary document, an acute record of a life and times, a revelation of unguessed-at experiences. Without an outside reader - that guilty eavesdropper - it has no function except as a passive confidant.

Human nature changes little, and you might think that keeping a diary must be a deep, unchanging human impulse in all literate people. Yet there is no evidence, for example, that the Greeks and Romans kept diaries. I asked my friend Peter Jones about this. He lectures in Classics at Newcastle University, and he took the question away and worried at it for a bit and then said: 'I don't know of anyone in classical times who wrote diaries; and if anyone did, none survive. I assume the reason is that the ancients were less conscious than we are of an inner world. They would have regarded daily events as too trivial to be worth recording. They did have a strong sense of a public persona, and anything worth writing down was assumed to be worth publishing - making several copies available for friends to read: probably using slaves as copyists.'

Samuel Pepys virtually invented the literary form. He clearly intended his diary to remain secret - at any rate from his wife - since it was written in code. Luckily for us, the code was easily cracked. Pepys used his diary to record his daily life, his accounts, the state of his marriage, his sexual peccadilloes; in short, his uniquely Pepysian-eye view. He also observed great events - most famously, the Great Fire of London.

It must be more than coincidence that John Evelyn's diary, albeit written over a far longer span (1640-1706) was roughly contemporary with Pepys's. Was it the uncertainty of the times that prompted them to conceal their private thoughts and at the same time furnish evidence of where they had been, and with whom?

Tony Nuttall, Professor of English at New College, Oxford, thinks the practice of keeping a diary may derive from the tradition of introspection and self- examination at the end of each day, as seen in Puritan autobiographies such as that of Richard Baxter. This, together with the practice of daily observations carried out by members of the Royal Society and early scientists, could explain the sudden onset of diary-keeping.

Why do so many ordinary people still - in this visual, non-verbal age - find the daily habit of diary-writing an outlet for emotional stress? At its simplest, the diary is a private and unchallengeable way of recording and decoding the world around us. It can fulfil this function for anyone from a lonely schoolchild to an international politician. A child's diary is perhaps the most secret of all, for in it the writer begins to explore who and what he/she is, taking the first steps towards independence of thought and action from parental control.

The betrayal felt by the adolescent whose parent reads his or her diary is justified. The politician has less reason to object, since politicians move in the public domain and know that their behaviour must be tempered by their visibility. Their diaries often seem to be written chiefly to ensure that history takes their point of view into account.

But neither impulse explains the folly of diarists contemplating a crime. The taboo against reading another person's private diary goes deep, but it doesn't stretch to the police; and many people have suffered the ultimate indignity of sitting in court hearing themselves damned by their own, most secret, words: 'I wish she were dead.'