Rembrandt, 'Self Portrait at the Age of 63', 1669

You are looking into the eyes of a man in the last year of his life. Rembrandt is 63 and this is one of three self portraits he painted in the months before his death in 1669. About 80 self portraits – including sketches, etchings and paintings – survive from his 40-year career, far more than any other painter of his time. Why did Rembrandt paint himself so often? And what was he thinking as he looked in the mirror?

Some self portraits, especially the earlier ones, were made to practise depicting different expressions and poses, or to experiment with lighting effects. Others – such as the one he painted aged 34 – were apparently intended to enhance his image as an artist. Sometimes he may have had an eye to the market, or even worked to a commission. At the time some wealthy patrons collected self portraits by artists. King Charles I owned several, including one by Rembrandt.

Here his intentions are not so clear. X-ray images show that Rembrandt did, at first, depict himself as an artist at work – hands open, holding what seems to be a brush. But he changed his mind, apparently to avoid distracting the viewer from the detail of his face. Indeed, it seems as though he has turned a spotlight on his features. The beam bounces off his high forehead and the tip of his nose; it catches on his mottled, pitted brow and the tufts of hair around his lips. By contrast, the dark, shadowy background is thinly painted, the texture of his robe and fur collar sketchy and his folded hands are blurred and indistinct.

It wasn’t only light effects that he used to emphasise his face. The paint here is different. It’s much thicker and more intensively applied; so much so that he appears to be modelling the skin in three dimensions. The sagging fold beneath his right eye is made with the swirl of a heavily-loaded brush. The blemishes on his forehead are formed of blotches of coagulated paint. He uses layers too: like skin, oil paint is semi-transparent, so Rembrandt builds up translucent layers and colours – the greys and whites, pinks, purples and pasty yellows which he sees in his own face.

The effect seems to be one of intense self-scrutiny, and many later writers and artists have regarded this, together with his other late self portraits, as part of a process of self examination – Rembrandt coming to terms with the approach of death, searching with unflinching honesty to know himself. In this portrait some see weariness and resignation in his dark, rheumy eyes, some a trace of self satisfaction, others the hint of a wry, knowing smile.

But, if we want to understand why Rembrandt made the painting we need to be careful. In the seventeenth century people had different ideas about self-analysis and how the mind works than we do now. And Rembrandt’s motives may have been rather more straightforward, driven less by soul-searching and more by a professional fascination with the textures, flaws and subtleties of the human flesh.

When he painted himself, he could study his reflection for as long as he liked – and with an intensity which might have been disconcerting for some of his clients, many of whom were the same age as him. So it seems as though Rembrandt was not only studying how time was ageing his own skin, he was perfecting how he might depict theirs. That mysterious look in his eye may not be one of existential angst, but that of a painter deeply engaged in the challenges of his art.