Really? Cezanne springs to mind as an example of a painter for whom this truism is plainly untrue. So, come to that, do Raphael and Rothko and just about any other artist whose reputation lies in the development of a consistent talent. Good curatorship means good editing: you are more likely to learn something about these painters from the exhibition of a dozen well-shown works than from a 300-picture blockbuster. If you emerge from the Tate's show of Barns-Graham's pictures feeling that your curiosity has been left unsatisfied, then the fault lies with the exhibition rather than with exhibitions as a whole.
More precisely, the fault lies with the work in this exhibition. If your first reaction to the name Wilhelmina Barns-Graham is a sheepish "who?", then a visit to Cornwall may explain why. Born in Fife in 1912, Barns- Graham moved to St Ives in 1940 and began to paint alongside such local giants as Ben Nicholson, Naum Gabo and Barbara Hepworth. Bravely (if, possibly, a little foolishly), the Tate has chosen to show Barns-Graham's work together with a selection of paintings and sculptures by her peers. The obvious intention is that this should place Barns-Graham's oeuvre in some kind of historical context, although there is also a faint whiff of revisionism to the Tate's agenda. Isn't it a pity, the show seems to say, that the work of an artist who painted with Hepworth and Nicholson should somehow, unlike them, have come to be unjustly overlooked.
The suggestion is an interesting one. Why should we remember the names of Naum Gabo and Alfred Wallis and not that of Wilhelmina Barns-Graham? Unfortunately, the answer is all too obvious from the Tate's show: namely, that while Gabo and Wallis worked like themselves, Barns-Graham, at various times, worked like them both (not to mention like Ben Nicholson and John Wells and Victor Pasmore).
Now, you may say that accusing Barns-Graham of painting like Wallis or Pasmore is about as useful as criticising Jacques-Louis David for painting like Poussin. Art, and especially the art of St Ives, is all about cross-fertilisation: Nicholson painting like Hepworth, Hepworth sculpting like Nicholson. The trouble is that Barns-Graham's borrowings from the group aren't so much cross- fertilising as self-denying.
Works like Suspended Ice and the small but vivid Zennor Rock - Rose III (both 1951) pulse with energy and talent; both show an extraordinary sense of abstract form, of the dynamic qualities of rapid brushwork. At the same time, though, there is the feeling to them of Woody Allen's Zelig, whose central character can only come to life by slipping into another character's role. Look around the Tate St Ives and you will quickly see why history has been less kind than it might have been to Barns-Graham's reputation: there's nothing that really looks like a Barns-Graham. Somewhere in all of this, you sense either a crippling reticence in the painter's character or a lack of faith in her own talent: neither of them likely to endear an artist to the history books.
This feeling persists in Barns-Graham's later works. Although pictures like Expanding Forms, Touch Point Series: Movement Over Sand Number 2 (1983-86) are no longer identifiably in the manner of any specific painter, they construct themselves all too clearly in terms of specific movements in contemporary art. There is no rule that says that pictures can't skip back and forth between genres, but Expanding Forms does so cold-bloodedly, grafting a hard-edged interest in geometric abstraction on to a taste for scumbled landscapes. The result is messy. More tellingly, it is unconvincing: you feel that Barns-Graham's heart either wasn't in the picture or that she was unable or unwilling to show it.
What makes all this particularly sad is a series of chalk drawings tucked away in the corner of the main room. Dating between 1951 and1982 these were clearly intended for private consumption. Freed of the need to appear in public, Barns-Graham shows a real flash of genius in these works. They make you wish that she could have taken it further.
`Wilhelmina Barns-Graham': Tate Gallery, St Ives (01736 796543) to 7 May