Ellsworth Kelly died yesterday. His brilliant work is impossible to capture in photos as he strived his whole career to achieve a complete unity of form, colour, and materiality. We could say that you cannot get more abstract than Kelly: his most famous works are shapes made out of aluminium, completely even and smooth in colour. None of the covert symbolism of Ad Reinhardt's ton-sur-ton crosses. None of the histrionics of Barnett Newman's single gesture on his Onements (nor, of course, of Newman's penchant for drama and pretentious references in the choice of titles). None of Rothko's hypnotic landscape-like depth. None of the industrial quality of Donald Judd. None of the geometric rigour of Frank Stella.

However, we could also say that because of this, Kelly was the most concrete of all abstract artists. His work needs to be seen. It cannot be expressed in a sketch, a photo, or a brief description. Its colours are saturated, dense, impossibly even. Others have worked with the idea of a shaped canvas - perhaps Stella most notably - but none as consistently as Kelly. In Kelly's work the aluminium surface is frame and canvas at the same time: shape and substance. Painting and sculpture. It is truly what Judd would call a 'specific object' that is as impossible to categorize with words as it is to record on film. The pieces are large, often in scale with the human body, allowing the viewer to lose himself in the flatness and intensity of their colour. As Kelly himself said, 'the form of my painting is its content'. Many XXth century artists would have liked to be able to say the same, but not many succeeded in this quest as much as Kelly did. In books and websites, his works are sometimes titled 'forms', and sometimes 'shapes' - I am trying to understand whether this is a mistake or a choice. In general terms, shape is defined as the 'outer appearance' of form (which is supposed to be the fixed, unchangeable, and radical base of the shape). As such shape is subject to change but also individual interpretation, whereas form is supposed to carry with it the oneness of Plato's ideas. In any case, Kelly's specific objects could also be interpreted as shapes, perhaps to insist on their possibility to be understood and misunderstood in different ways.

Can we then charge Kelly's work with being 'literal' and lacking 'presentness', as Michael Fried did with most minimalist art? Can we apply Fried's critique to it - namely, can we say these are 'theatrical' pieces that depend on the viewer's presence to have any meaning at all? Perhaps not. While it is true that Kelly's shapes defy digital diffusion and can only be seen in person, this does not mean that they depend on the viewer. On the contrary, their strong formal aspects make them, if anything, very complete in themselves, almost monumental in their precision, smoothness, size, and geometry. They are mysterious and somewhat impenetrable, they exist in and of themselves with a solidity and absoluteness that seems to ignore - rather than challenge - the viewer. They can be interpreted as fragments, of course, and as such they can be seen as 'incomplete'. However this is not at all the impression that they give in real life, as their scale makes them clearly alien to the everyday context. They are obviously manmade, yet also timeless, non-referential.

In this he perhaps managed to do even better than one of the people who inspired him the most as a young artist: John Cage. When Kelly lived in Europe in the 1950s, he looked up to Cage as a mentor and friend and indeed there are similarities between their researches. The attempt to make of art only a frame is something Cage and Kelly shared, albeit in very different ways. Cage himself told an interviewer in 1978 that decades before de Kooning would tease him saying 'it's not like I can do art by simply framing some breadcrumbs', to which Cage took some breadcrumbs from the dinner table, framed them with his fingers, and retorted that it's exactly what one should do. However, Kelly's frames do not play with the aleatory, neither they depend on chance. Kelly made of the frame his main job and that's what makes his work so elusive - but perhaps also so architectural (not theatrical: architectural). On the other hand Cage's music embraced a certain unselfconsciousness and playfulness that are very much part of Kelly's work as well. Without ceasing to be rigorous, Kelly's art has a refreshing effortlessness to it.

Obviously in over 70 years of career Kelly produced works of different kinds but we mainly refer here to his monochrome shapes, which constitute the bulk of his work and he produced in a consistent manner since the late 1950s, never abandoning a clear line of research and yet never repeating himself. The relationship between form and colour in these works is extremely interesting - why should the red panel be triangular, and the yellow rectangle with two filleted corners yellow? Is it an attempt to make different forms of perception coincide syaesthetically, or is it the ultimate demonstration of the fact that our understanding of colour is historically constructed? In this, his monochromes are much more challenging than others (from Ryman to Klein).

He often worked with black but as you can expect his black artworks (paintings? sculptures? objects?) are never really square.