Sometimes, I obsess about small, apparently unimportant, things. Elspeth, my wife, says that this is why I have risen so high in my chosen profession; it is a civil servant’s job to obsess about things others pay no attention to.

For the most part, she means this observation to be humorous.

I am grateful for her tolerance but we both know, that buried in the flesh of her remark is a tiny splinter of resentment at my distraction that she cannot remove and which neither of us can completely ignore.

I am aware that I spend too much time inside my own head. I impose structure on the most inconsequential of events. I find spontaneity suspect and I tend to treat happiness as a temporary aberration from the norm.

I am not an easy man to live with.

Yet, Elspeth has spent the last twenty-four years at my side. I take this as a sign of her love for me.

If I were an American, perish the thought, I would probably have been diagnosed as having Obsessive Compulsive Disorder by now and would either be in therapy or be loaded to the eyeballs with mind-altering drugs.

Fortunately, I am a Scot of a particular class and so my obsessions are seen as mere peccadilloes as long as I fulfil my duties to society and uphold the honour of the family name.

Today, my obsessions have brought me once again to the National Gallery. I come here several times a week to find moments of calm away from the frenzy that is Whitehall immediately after a change of government. The quiet focus of the place allows me to slough off the cares of the day and listen to myself.

Sometimes, like today, I will arrange to meet Elspeth for lunch in The Dining Rooms. But first I always spend time alone in the Gallery, looking into the faces of the great and the good that hang there as if there is something important that they can tell me.

For the past few weeks, every visit has ended with the same painting: Jan van Eyck’s portrait of his wife, Margaret. I do not yet know why this painting is important to me but I have learnt that my small obsessions are the way that I reveal to my mind truths that my heart already understands.

Van Eyck’s portrait of his wife shows her strength and intelligence in an honest and bravely unglamorous way but it is not what is on the surface that fascinates me about this portrait.

It seems that infrared reflectography shows extensive pentimenti in the painting. The National refers to them as “underdrawings”, perhaps to avoid the now frowned-upon use of a foreign word, presumably on the grounds that only an educated reader would know what it meant. I find this attempt at egalitarianism distasteful as the word selected reminds me of Y-fronts and singlets. But I digress.

The point is that the painting that we see with the naked eye today is not all that Van Eyck painted. It seems that he originally presented things one way and then painted over them to present them in another.

I suppose it is normal enough for a painter to change his mind but what puzzles me is why he made so many changes to the portrait of his wife. She must have been available to him as a model whenever he needed her to sit and he clearly knew her well, so why should a man who sees so clearly need to make so many revisions?

I have no gift for portraiture but if I did, I wonder how well and how decisively I would paint Elspeth?

Even after all these years, there are many things about her that I do not understand. Perhaps the greatest of these is what it is about me that stirs her affection.

I asked Elspeth about this quite directly a few weeks ago. We were celebrating our wedding anniversary with a pleasant meal at The Grill in The Dorchester. We go there every year. I enjoy the lamb that they serve: it’s Welsh and organic and reminds me of how meat used to taste when I was a boy. We’d made our way through a surprisingly good bottle of South African Merlot and it seemed to me that this would be the perfect opportunity to discover why Elspeth endures me.

Looking back, I can see that I wasn’t setting a particularly celebratory tone and that an outsider might even have concluded that I was challenging Elspeth on her poor judgement.

Elspeth waited patiently while I explained all the things that made her involvement with me difficult. Then she put aside her knife and fork and scrutinised me carefully, as if assessing my state of mind. After a few moments, she spoke.

“Well, Alistair,” she said, “you can indeed be an infuriating man: socially myopic, emotionally distant to everyone but me and our children, and prone to obsessive behaviour that borders on the compulsive. This latter I have learnt to tolerate. After all, I have been an obsession of yours for many years now.

As for what I see in you, at first, I was attracted to your good looks and your ability to focus on an objective until you had achieved it. It wasn’t until much later that I realized that I was the objective you were trying to achieve. Even then I assumed that your objective was only to have sex with me.

The sex is very good, by the way. I believe it’s because you don’t think about it or question it; you need it and you take it and in the process, you give everything that you have. Many, many times I have lain watching you sleep after sex and smiled because only I know what a fierce and creative lover you are.”

This took me completely by surprise. I have always found sex with Elspeth compelling. I had not considered that she might be quietly evaluating my performance. It should not have surprised me; quiet evaluation is part of Elspeth’s approach to life.

I made as if to comment but Elspeth, said, “Do not interrupt, Alistair. I want you to listen, then we won’t need to talk about this further. You needn’t look so surprised at the fact that I enjoy you as my lover. Sexual attraction was the start of everything for you and me. You changed my narrow little virginal world.

After you took me the first time, in the bottom of your father’s rowing boat, still moored in the cool darkness of the boathouse, I felt as if I had become a boat myself: floating, tethered to the world by the thinnest of ropes and ready to launch into deeper waters.

I was sad because I thought your objective had been achieved and your attention would be snared by some other obsession. But you have never moved on. You have a good heart and you have given all of it to me.

Sometimes, that is a burden I would like to set down for a while. Most of the time, I see it for what it is, the rock on which I stand.

Now do stop going on about how awful you are before I decide to believe you. Order a good Port and then take your ageing wife to bed and help to launch her once more.”

Now, with this memory fresh in my mind and Van Eyck’s portrait of Margaret in front of me, I suddenly understand why Van Eyck made so many changes and why I have returned to this painting again and again since that Wedding Anniversary dinner.

I walk away from the painting. I will not need to visit it again.

The Dining Room at the National is flooded with daylight. I use it to study Elspeth’s face during our meal.

I try to see Elspeth as a stranger might: a woman in dignified middle age with good bones, a dancer’s posture and a demeanour that suggests strength without making her unapproachable.

I too see those things, but they are only a fraction of what is there. Our history grants me the infrared vision that lets me see the many pentimenti that form the image of Elspeth that dominates my heart.

Love has over-painted lust. Age has begun to add craquelure to youth. Beneath the face of the strong matron, I see the proud mother, the pregnant wife, and the young girl who lay with me in a rowing boat many years ago. I see them all at once. To me, Elspeth is all the things she is now and all that she ever was during the years we have been together.

The parts of the portrait of Elspeth that have been most worked and reinforced in my imagination are the ones that tie me to her: her ability to see the beauty in the world, her excitement with ideas, her impatience with stupidity, her anger at injustice, her love for me.

I realize that Elspeth has stopped talking and is looking at me closely.

“I love you, Elspeth,” I say. “You are everything that I want; everything that I need.”

I lack the words to say more. Having an epiphany is not, it seems, the same as being able to share one.

Elspeth puts her hand over mine and smiles. I realize that she is welcoming me to a truth that she has long understood and is pleased that I have finally discovered.