We don’t think too much about paintings happening over time. For most of the history of painting, the making of it is not what’s being brought to our attention. A painting exists as a sealed entity, always ready, always fresh for us to view it, again and again and again. Unless something bad happens, it remains essentially the same. Finished. Even if we don’t like the work, we assume it’s supposed to look like that.

But work takes time: a day, an hour, a month, 40 years. Things can get in the way: an artist’s attention wanders, some niggle keeps the piece from coalescing, a better idea appears or the money just runs out. Sometimes history intervenes. The enemy is at the door and the work must be abandoned. Death is the great unfinisher.

We prefer things finished: books and buildings and symphonies. When they are not, we tend to name them fragments. It’s a soft, seductive word. From a fragment, you can spin off anything. A finished work is less compliant. It’s a fixed, end state. So it’s easy to forget that finishing is a time-based experience. The Finish is where you arrive at. It happens only once. Or you don’t arrive at all.

A small show of paintings currently on at the Courtauld in London, “Unfinished”, presents us with a clutch of works that have been arrested, truncated, mid-process. The collection is very random. It doesn’t aim to tell us about any intervening life stuff. It is practical and material, about how artists work. What is clear is that the artists who abandoned the works in the show fall into two categories. There are those who regard paint as matter: working all over the canvas, they put on as much or as little paint as they felt the job demanded. Matisse, Derain, Cézanne (“Turning Road”, 1905, detail above) fall into this category. Daumier too. Daumier always looks unfinished. That’s the beauty of him. Then there are those who regard painting as space to be filled, here represented by Rembrandt, Piero del Vaga, Mantegna. This is more mysterious. Mantegna’s print, “The Flagellation of Christ” (1465-70) goes blank at the top and left-hand edge yet the rest is highly worked. Seen like this, the effect is weirdly mechanical, as if he was just filling in. Half the prints attributed to Mantegna were left unfinished. If one didn’t work, he tried another.

In terms of art history there’s a very clear shift. If you whizz around the rest of the Courtauld, you can see that in work from around 1900 onwards, the idea of a finish becomes fluid. It is a continuum: paint as matter starts to look exhilarating, things loosen up, the sketch makes an appearance. Matisse’s “Woman in a Kimono” (1906) sets a figure with scrappy, daubed flowers onto raw canvas. It is not in the show but looks equally unfinished. When artists get interested in painting as a process, in speed and time, then the idea of finish as an end state becomes malleable. Painting becomes a record of the urgency of being, of presentness: as temporal as it is visual and as visual as it is temporal. Things can come to a stop.

As an artist, I have never had issues with finishing. I don’t have a studio full of things half done, things I can’t let go. Mainly I make sculptural objects, films or take photographs, so I know what finish feels like in many forms. I know when I get near and I never tinker after the fact. Working on my book, “The Iceberg”, was fundamentally different. “The Iceberg” is a memoir on art, work, death and language. It is highly concerned with the urgency of being. It is an account of a small family unit—mine—during the period, lasting just over two years, between the diagnosis and death of my husband, the art critic Tom Lubbock. The work and its duration, its start and finish, were more intimately connected than in any work I have ever made.

“The Iceberg” went through 13 numbered drafts and countless unrecorded ones. I started numbering when I understood that what I was writing was indeed a book: something that required completion—a finish—rather than bunches of words collated together. Once I understood that, it was simply a matter of time. I didn’t know how other people finished books. I had never written one before. But I realised that completion in a book implied an arc, a shape, or a series of interlocking shapes and there was something about the process of stripping, shaping and restructuring these shapes that felt familiar. It felt like film editing: visual as well as temporal. With an edit, you are always keeping an eye on the whole. With each draft I came nearer.

Functionally, the point of finish was a light step, like stepping off a kerb. I sort of missed it. I had simply reached a point where there was nothing left to do. Emotionally, the impact was not light at all. The work that had been my motor for so long took many, many months to leave. With a finished work, there is no returning.

Unfinished Courtauld Gallery, London, to September 20th