Gerhard Richter’s life, like the country of his birth, was divided in two by the turbulent 20th century. Now aged 92, he was born in the easterly German city of Dresden the year before the National Socialist Party came to power. He grew up under Nazism, joined the Hitler Youth like practically all his generation, witnessed from a distance the bombing that obliterated his home town, and reached adulthood in the new-minted, Soviet-controlled German Democratic Republic — the DDR. His experiences under two dictatorships had left him with an almost phobic loathing of dogma. ‘By the age of 16 or 17,’ he said many years later, ‘my fundamental aversion to all beliefs and ideologies was fully developed.’

In the summer of 1961, the 29-year-old Richter packed a bag and quietly made his way west (avoiding Berlin, because the trains were too closely watched). Had he hesitated for a few weeks, he would have missed his chance: the Berlin Wall went up in August, and the border between the two Germanies was closed for a generation.

This was the liminal moment, a crossing-over into a new country and a new life. He settled in Düsseldorf and, significantly, destroyed almost everything he had produced up to that time. It was as if the innovative West German painter Gerhard Richter were a different artist from state-employed East German painter Gerhard Richter — the shared name some kind of bizarre coincidence. His existence as a free artist begins in Düsseldorf.

So it is fitting that a major survey of Richter’s career is about to open at the Kunstpalast Düsseldorf. The exhibition is called Verborgene Schätze, ‘Hidden Gems’, because it consists almost entirely of little-seen works drawn from private collections in the Rhineland. But the title of the show could also be read as a reference to Richter’s oeuvre more generally, which is famously and bewilderingly diverse — yet at the same time imbued with unanswerable questions posed by a man who has been a conscious, conscientious observer of the human condition for the greater part of a century.

Richter is, of all contemporary artists, perhaps the hardest to know, not least because his own comments about his work and art in general are often contrary and contradictory, hard to square with his vast and varied output.

Photography is the thread that runs through nearly all Richter’s work. Photographs became central to his thinking and his process soon after he began work in Düsseldorf. ‘In 1962 I found my first escape hatch: by painting from photographs I was relieved of the need to choose or construct a subject.’

Richter began to make painted enlargements of seemingly random snapshots — a chair, a Flemish chandelier, clouds, family groups. The copies were accurate in that they looked like photographs — but they were not photorealistic (that term was not coined until the end of the decade). He undermined any suggestion of painterly virtuosity in his copies by making the images slightly smudged or blurry.

The softness means that a viewer can see straight away that the works were made with paint, although they are also recognisably photographic images. The effect is pointedly ambiguous: are these pictures both paintings and photos? Or neither photos nor paintings? Richter said that he aimed to ‘use painting as a means to photography’. Whatever he meant by that explanation, the photo-paintings are a kind of Magrittean pipe. They say ‘Ceci n’est pas un Polaroid’ — which is perhaps what Richter was driving at when he remarked in passing that he was a Surrealist.

As Richter’s reputation grew, the blurred photo-paintings came to be seen — somewhat to his irritation — as a signature style. (‘Blurring is not the most important thing,’ he said, ‘nor is it an identity tag.’) But he persevered, perhaps because the blurring proved so fruitful and so fraught with meaning.

There is an early work called Helga Matura, derived from a newspaper image of a smiling woman. The original article made clear that she had become the victim of a sordid murder. In this picture the effect of the blurring is sinister, somewhat ghoulish. Oddly, and prophetically, it resembles the uncertain, unreal images produced by artificial intelligence. An algorithm, after all, is like the camera in that it doesn’t know what it is looking at, and cannot form a judgement about what it has captured.

Then, by contrast, there are later photo-paintings such as Betty, perhaps Richter’s most famous work. This portrait of Richter’s 11-year-old daughter is derived from a colour photograph that Richter took himself. The blurring is subtle, well nigh indistinguishable from a camera’s soft focus. The twisting pose of the young girl is reminiscent of the classical contrapposto, but in this photographic context it looks as if she was startled by something behind her just as the camera clicked. Again, the image manages to be painterly while retaining the air of an accidental photograph.

There is something of the Old Master about Richter. Partly it is his ambitious exploration and reimagining of the time-honoured genres — portraiture, landscape, still life — but it also has to do with his commitment to the daily grind of making art. He has said that he recognises ‘no idea that I serve and am known for… no rules that regulate the how, no belief that gives me direction, no picture of the future, no instruction that produces an overly ordered mind. I acknowledge only what is.’

And yet he does have a creed that is all his own: it comes out of a tube and smells of linseed oil. Paint is Richter’s theology. ‘Once there are no more priests and philosophers,’ he has written, ‘artists are the most important people in the world.’

At the same time, Richter is naturally a product of the experimental, boundary-breaking age he was born into. The grandeur of his large abstractions suggests some kind of parallel with other 20th-century masters of non-figurative or nature-adjacent painting. In individual works by Richter it is possible to see a kinship with the epic seascapes of Zao Wou-Ki, or the impastoed petrified forests of Pierre Soulages, or Joan Mitchell’s almost-trees, painted as she confronted death in her windowless studio.

It is not that Richter is influenced by these contemporaries and predecessors; it is more that the impassioned need to express what is beyond words somehow leads to cognate visual solutions, as if all the most thoughtful artists of the age were pursuing the same global archetypes of representation.

Many of Richter’s large abstract canvases were produced in the 1970s and 1980s. They appear at first glance to be the least photographically led strand in Richter’s work, but often they are based on enlargements of previously worked areas of canvas: they are microscopic close-ups of other Richter paintings.

More than that, there are moments when the abstract canvases seem to contain echoes of earlier, figurative photo-paintings. Take I.S.A., a large, predominantly yellow abstract work that is part of the Hidden Gems exhibition. The black wedges and vectors at the right of the canvas could be an unconscious echo of the forms that make up the diving Stukas in a photo-painting from 1964. Richter has said, half in jest, that he is ‘an expert on aircraft’, and objected when an interviewer suggested that his paintings of warplanes might be anti-war statements rather than the ‘pure picture’ that he aspires to. The angular geometry embedded in I.S.A. suggests that he has simply always liked these kinds of shapes.

In 1986, Richter found a new way to combine paint and photography. Instead of turning to photographs as a source, he began using them as a canvas, as the actual ground onto which he painted. These are the Übermalungen, or ‘overpaintings’. Unlike photo-paintings, they are subject to chance, which is ‘the found object, which you then accept, alter or even destroy — but always control. The process of generating the chance event can be as planned or deliberate as you like.’

The photographs are of his own choosing, of course, and the pigments he used to paint over them were the leftovers of a day’s work in the studio. So a certain degree of curation went on, however unconsidered. This must be what Richter means when he says that chance can be planned for.

To make the overpaintings, Richter pressed photographs into the paint on his used squeegee, or wiped them across it. The sheer capriciousness of the method throws up strange things: a smear of paint obliterates the face of a bridegroom, replacing it with a Bacon-like distorted head. Lateral green stripes across a photo of Richter’s wife Sabine make it appear that she is peering fearfully through a strange, psychedelic Venetian blind. Traces of red pigment make her eyes look bloodshot, as if she has been weeping. Suddenly she is a Dora Maar figure, a femme qui pleure. Overpainting’s element of chance has done its work: it has fundamentally altered the meaning and the emotional import of the original photograph.

The overpainted photographs, like the photo-paintings, are all drawn from Richter’s ‘Atlas’, a carefully collated scrapbook of photographs that Richter has been keeping and augmenting for many years. This collection consists of press clippings, domestic snaps made by Richter himself, evidential shots from crime scenes — plus the kind of dusty old photos that, as one critic has pointed out, most families consign to a shoebox at the back of a cupboard. Because the items in the Atlas are so numerous, there is room for randomness, or for the impression of it. The Atlas can be a bottomless well of found things when only one in a thousand images catches the artist’s eye.