The Courtauld Cézannes, Courtauld Gallery, London

A new exhibition focusing on his lesser-known works shows the Post Impressionist at his most questioning

Oddly missing from the Courtauld Gall-ery's show, The Courtauld Cézannes, is the man who gave his name to both: namely Samuel Courtauld, the industrialist turned collector whose millions, made from rayon, brought Impressionism and Post-Impressionism to Britain.

It is easy enough to see why Courtauld might have snapped up some of the things he did – Renoir's La Loge, say, or Manet's A Bar at the Folies-Bergère – but his Cézannes are a different matter. Bought mostly in the 1920s, these were difficult pictures, admired by a handful of cranks of the stamp of John Maynard Keynes. Forty years later, when the National Gallery finally got around to acquiring Cézanne's Les Grandes Baigneuses, the purchase caused such outrage in the shires that the picture had to be shown behind a protective screen. So Courtauld's early fondness for the provençal painter needs looking into, which this tight and clever show does not.

Although that is my only moan about it, it is nonetheless worth mentioning. For the show's tightness and cleverness, we have also to thank Samuel Courtauld, whose admiration for Cézanne seems to have been as much a matter of logic as of passion. While collectors such as the Cone sisters of Baltimore tended to go for one kind of work by the artist, Courtauld bought encyclopaedically: landscapes, still lifes, interiors, portraits, genre paintings, oils, watercolours and drawings made across Cézanne's career. He also bought Cézanne's letters to the artist Emile Bernard, among them the famous one whose words of advice are known to every art history undergraduate: "Treat nature in terms of the cylinder, the sphere and the cone." That it seems likely the older painter was actually suggesting a handy mnemonic for dealing with perspective hasn't stopped Cézanne's phrase from being held accountable for Cubism – a touch far-fetched when seen with other letters in this show. ("The Louvre is the book wherein we learn to read," says one: hardly a manifesto for revolution.)

Now, an exhibition of work by a single artist amassed by a single collector is a very specific type of show, reflecting as it does the taste not of one man but of two. Although Courtauld, as I say, bought widely, you can't help feeling that a single thing connects all the paintings and drawings he collected, that there is a specifically Courtauld-ish Cézanne. And, broadly speaking, the quality that links the works in this show is a workmanlikeness, found at those moments in Cézanne's ever-changing oeuvre when he is trying to answer difficult questions he has set for himself.

Because of this, the works that seem most typical of Courtauld's taste aren't the formally secure Pot de primevères et fruit or the harmonious Le Lac d'Annecy, but less obviously successful pictures such as L'Etang des Soeurs, Osny, près de Pontoise. That this painting's strong diagonals are laid on with a palette knife rather than a brush may, as has been suggested, be down to Cézanne's rejection of academic method, although I doubt the artist would have said so himself. ("It is the art critic [who] expresses himself in abstractions," he sniffs in a letter to Bernard. "Don't be a critic. Make paintings.") Instead, Cézanne seems to be dealing with his latest problem by treating it as manual rather than cerebral, its solution to be found with a tool rather than a brush.

Maybe this is true of Cézanne as a whole, and maybe it is that which Courtauld saw. The artist's greatest works are not his best, certainly not his easiest or most accomplished. The images that caught Courtauld's eye are the problematic ones whose subject is problem-solving.

Thus Nature morte avec l'amour en plâtre, a still life with plaster cupid, whose point is that its composition so very nearly falls apart. Fumbling for a new equation between surface and depth, mark-making and representation, Cézanne almost loses it. And thus, too, Une Cabane, the artist's little-known drawing of a shed, where the composition is built up as though it were brickwork or cut stone, plane upon plane, mark upon mark.

What emerges from this show – for me, at any rate – is an unexpected Cézanne, whose approach to his art is that of an engineer rather than of a proto-Cubist theoretician. You can't help feeling that we owe this revelation, in part, to Samuel Courtauld; that his is the eye that saw it, even if he didn't know it. For this and much else, thank you, Sam.

Courtauld Gallery, London WC2 (020-7848 2526) to 5 Oct