The first colour charts were unsystematic. They were based directly on commercial colour samples. They were still related to Pop Art. In the canvases that followed, the colours were chosen arbitrarily and drawn by chance. Then, 180 tones were mixed according to a given system and drawn by chance to make four variations of 180 tones. But after that the number 180 seemed too arbitrary to me, so I developed a system based on a number of rigorously defined tones and proportions.

It's that same quality I've been talking about. It's neither contrived, nor surprising and smart, not baffling, not witty, not interesting, not cynical, it can't be planned and it probably can't even be described. It's just good.

When I first painted a number of canvases grey all over (about eight years ago), I did so because I did not know what to paint, or what there might be to paint: so wretched a start could lead to nothing meaningful. As time went on, however, I observed differences of quality among the grey surfaces - and also that these betrayed nothing of the destructive motivation that lay behind them. The pictures began to teach me. By generalizing a personal dilemma, they resolved it.

I first came up with the idea for the colour-chart pictures back in 1966, and my preoccupation with the topic culminated in 1974 with a painting that consisted of 4,096 colour fields. Initially I was attracted by the typical Pop Art aestheticism of using standard colour-sample cards; I preferred the unartistic, tasteful and secular illustration of the different tones to the paintings of Albers, Bill, Calderara, Lohse, etc.

Nature/Structure. There is no more to say. In my pictures I reduce to that. But 'reduce' is the wrong word, because these are not simplifications. I can't verbalize what I am working on: to me, it is many-layered by definition; it is what is more important, what is more true.

Painting pictures is simply the official, the daily work, the profession, and in the case of the watercolours I can sooner afford to follow my mood, my spirits.

As a matter of fact, it was only through the dealer Fred Jahn that I succeeded in overcoming my reservation about the works on paper and exhibiting them. Added to this, of course, was the fact that after ten years I could see the watercolours in a different light, and in conjunction with pictures painted afterwards, they had at least become more comprehensible to me.