Balance and depth in a little-known master

If Masaccio was one of those artists whose work succeeding artists found they could not ignore, it was because of both the downright force and the sensitivity of his statements in paint. ''Statement'' is a good word for this deeply respected early Renaissance painter's art, because there is nothing tentative about it; it is undeniable and unmistakable. It has the apparent simplicity and directness of art dictated by bold certainty.

But the word ''simplicity'' could be deceptive if it gave the idea that he somehow expunged every complexity or nuance from his style except for his well-known, and powerfully new, ability to give figures solidity and three-dimensionality. This most obvious feature of his work - of which ''The Tribute Money'' is the crucial example - is undoubted. With their weighty and firm stance, his figures are often said to be bound to the earth differently from any figures painted before them.

Unlike the typical people in medieval pictures, their feet ''grip the ground'' (as Philip Hendy has written): they don't seem to float lightly just above it.

Obviously also, Masaccio was influenced, as few painters had previously been, by sculpture. The sculptor closest to him was his Florentine friend and contemporary, Donatello, whose reliefs reciprocally brought sculpture very near to painting. Two earlier Italian sculptors, of the 13th and 14th centuries, might be seen as links to the other strong sculptural influence on Masaccio. They were Nicola and Giovanni Pisano, whose work tended to be nobly heroic and classically vigorous. Their eyes, like Masaccio's, were turned towards late Roman sculpture with its grand carved expressiveness and solemn dignity.

In ''The Tribute Money'' it is the central gathering of disciples around Jesus and Peter which owes most to antique sculpture. As a close-knit body of people, it has its own identity and mass. It surely shows the influence of carved figures crammed together by the tight requirements of a scene on a Roman sarcophagus. This crowding of Masaccio's figures, all virtually on the same level patch of ground, hardly suggests the ''space'' for which the painter is frequently eulogized. In fact, it is a dense interaction of humanity stirred by a challenging idea, concentrated by it.

It is also interesting to consider that Masaccio's use of perspective in ''The Tribute Money'' does not make for a large arena in which his figures are freely ranged. All these figures are in the foreground of the picture, each individual almost vying for prominence. They are like a frieze. No part of the narrative takes place distantly.

The lines of perspective of the building on the right culminate in a ''vanishing point,'' which happens to be the emphatically close-up head of Jesus. In other words, the sense of depth and space in ''The Tribute Money'' is actually countered by the row of foreground figures. The result is a remarkably subtle hint of great unseen depth behind them, allowing the mind's eye to assume vividly that they have rounded dimensionality, backs as well as fronts. This is also the very thing forced on our attention by the vigorous modelling of each figure, as if they were statues.

The very strong light which makes this modelling so powerful, and the deep shadows resulting from it, are a great source of expressiveness in the picture. Each face is structured by fierce light and shadow. Far from ''restraint'' - a word applied by some art historians to Masaccio - his works have a brooding intensity. The tension of his figures electrifies a moment and a place with sharp turns of hand, body and eye.

The eyes of the disciples here pierce like questions (they are similar to the eyes of people in Giotto's earlier paintings). Gestures are blunt and solemnly masculine. But there is also a definite grace of movement and posture to be found in all this strictly punctuated eloquence. There is no distracting prettiness or incident, no playfulness - but even the hills, which at first seem all ''bone and sinew'' - are, nevertheless, like the figures in front of them, strangely soft in texture. How it is possible for Masaccio to have painted his world with such monumental rigor and yet retain this gentle quality - how his light could be harsh and benign simultaneously - is one of the enigmatic secrets of this far from obvious painter.