I glimpsed him only fleetingly last Saturday morning as I sped in a car through the Derbyshire dales. Yet the picture he presented was as arresting and intriguing as any portrait by Rembrandt or Vermeer. Wearing a dark suit and tie, he sat stiffly on a bench in a village churchyard, seemingly oblivious to scurrying black clouds that were more redolent of January than July. Scattered around on the turf near his bench were bouquets and wreaths. Their fresh blooms suggested that a burial had taken place that morning. But of other mourners there was no sign.

I have no idea who he was. I don't know whether he had just lost a wife, parent, child or friend. I can't even be sure that he had attended the burial. Perhaps he was just a man in a suit who liked to sit and brood alone in country churchyards, like Thomas Gray.

But this I know, because from time to time I've also sought the solace of churchyards. He was wrapped in memories. Going through his mind, bidden or unbidden, jumbled or etched clearly, were remarks, sights, sounds, glances, gestures, embraces, recriminations, reconciliations, first meetings, last goodbyes, moments of inestimable joy and intense grief - perhaps stretching back for decades.

Our memories define us. They inspire and comfort us. And, in some respects, they trap us. Our inability to shake off the past, especially as we grow older, shapes our present and future behaviour. That goes for nations as much as individuals. To cite an obvious example, you can't fathom what makes modern Israel tick - if it does tick - without reading the Old Testament: a collection of tribal memories going back 3,000 years.

But what if our memories are “playing tricks”? If what we recall is a distortion or even a contradiction of the facts, isn't it dangerous to allow our memories to sway us to the extent they do? As a child I once spent a ghastly holiday on a caravan park in North Wales. The rain pelted down every day for a fortnight. The murk was impenetrable, mud knee-deep, latrines abominably blocked. Imagine the Glastonbury Festival without so much as a remedial spliff. Because of that one horrible memory I didn't set foot in North Wales for 30 years. Then some business took me there, and I was bowled over by the grandeur of the mountains. I realised that my memory of the place had been not only ludicrously subjective and partial; it had also unfairly discoloured my view of an entire country.

This misleading memory stunted only my enjoyment of Wales. A sad state of affairs, but not life-destroying. Misleading recollections with more serious consequences are scrutinised in a fascinating report issued last week by the British Psychological Society. As is suggested by its title - Memory and the Law - it looks specifically at how witnesses' memories help to determine criminal trials. And it draws a startling conclusion: uncorroborated eye-witness testimony cannot be relied upon.

That's especially true, the report says, in the case of witnesses who are very old or very young. But even middle-aged adults of sound mind, recounting memories they honestly believe to be true, are not to be trusted. “There is no such thing as a true memory,” says Professor Martin Conway, the report's main author. “It is a record of experience - but is your experience a true record of reality?”

Hmm. That's a deep one. What, you ask, is reality, if not the world as we experience it? And if you didn't ask, countless generations of venerable philosophers have asked the question on your behalf. It's one of the knottiest dilemmas of human existence. On the face of it, Professor Conway is right. I came to the same conclusion five years ago, when I wrote a centenary history of the London Symphony Orchestra. Hundreds of anecdotes and legends had accumulated over the decades around that mighty musical combo. I tried desperately hard to check their veracity and establish an “authorised version” before shoving them in the book. Hopeless! If 100 musicans overheard a fracas involving an egotistical maestro and a preening soloist (and they usually did), I would be given 100 different accounts of “what actually happened”. I came to realise that (as the old song suggests), the phrase “ah yes, I remember it well” is the world's most frequently-uttered unintentional falsehood.

But if there is no such thing as a true memory, there is also no such thing as a false memory that does not, in some way, reveal a truth - usually about the person retelling it. What we, as individuals and nations, believe about the past is usually what we want to believe, even if that involves some conscious or subconscious warping of the facts.

Great painters, authors and composers often “mythologise” reality: selecting some facts, discarding or distorting others, in order to highlight what they maintain to be a greater truth about the human condition. In our small way, all of us are similarly mythologising our own lives. By selecting and sorting our memories (even if we don't know we are doing it) we are attempting to make sense of the world, to discern some coherent pattern to our experiences. When I compared the sight of the man in the churchyard to a portrait by Rembrandt or Vermeer, I wasn't being facetious. I really believe that when we sit and remember, we are devising our own “picture” of reality in the same way that great painters artfully arrange the world on their canvases.

I also believe that if there is any immortality available to human beings, it depends on how we “live on” in other people's memories after we are gone. “If that's the case, you'd better choose some friends who have good powers of recall,” a colleague quipped when I mentioned this hypothesis. I disagree. I think my posthumous reputation would be greatly enhanced if my nearest and dearest generously developed a highly selective amnesia.

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