When church authorities rejected Caravaggio's work on the basis of indecorum, they generally cited such details as the obvious coarseness and human imperfection of his models. His apostles and saints were clearly the common people, with dirty feet and lived-in faces. All of which was, up to a point, thoroughly in line with the policies of the Counter-Reformation. But Caravaggio pushed matters further. His Mary in the Death of the Virgin looks bloated or pregnant. Different accounts identify the model as his friend Lena the prostitute, or, less plausibly, as the body of a prostitute fished from the Tiber. In some cases it seems likely that official disapproval relates to unstated sexual matters: that the angel guiding St Matthew towards writing the Gospel was just too intimate and physical a presence, for example.
In Pictures and Passions, his recent book about images of homosexuality in the visual arts, James Saslow (who, presumably thinking of the painter's documented relationship with Lena, describes him as bisexual) argues, not unreasonably on the evidence of their work, that Caravaggio and many of his contemporaries and imitators catered for a distinct "homosexual market", the economic crucible of an emerging subculture. It is certainly conceivable that he utilised a network of contacts, perhaps stemming from his first major patron, Cardinal Francesco del Monte.
But the puzzles about Caravaggio's work run deeper than his sexuality or the murky details of his misfortunes. To view the work as the expression of a covert homosexuality doesn't, Barsini and Detoit argue in their professedly speculative study, solve its riddles at all, as is the case, to some extent, with Michelangelo. They proceed, in a complex series of interpretations, to put Caravaggio on the couch, addressing themselves to certain puzzling aspects of the paintings. These include the patterns of crossed, misaligned glances, of odd gestures and attitudes, of curious discontinuities in narrative scenes that are otherwise meticulously conceived and executed.
Caravaggio worked directly from life, without preparatory drawings, an unorthodox practice at the time. He also tended to organise his compositions in a single plane of action - one dismissively characterised as a dark, cramped cellar. As Sergio Benedetti has observed, he usually edits the cast of any particular subject down to a minimum number of figures, and then pushes them together in a confined setting, so that they seem to strain at the limits of the picture, as if they are going to burst out.
He even uses visual tricks specifically to heighten this sense of compression. Or, again, there is his technique of pushing us, the viewers, right up against the figures from close, disorientating viewpoints. For the time, these positively cinematic effects are extraordinary on a technical level. But they are also used towards psychologically complex ends.
Caravaggio, Frank Stella has written, "was better at creating internal space, space among the figures constituting the action and subject of his pictures, than anyone who came either before or after him." This is why the interaction between his characters is as electric as, to cite an example in another form altogether, the intersecting voices in the quartet in Verdi's Rigoletto. The lines of presence, sight and gesture in the paintings cut through and across each other as incisively as the dazzling interplay of the different musical voices.
Analysing these claustrophobic dramas, Bersani and Dutoit pursue ideas of the ambiguous "soliciting gaze" and gesture in the paintings, of the precise relationships between figures, of the importance of the notion of betrayal, and of his practice of having subjects address us directly, and provocatively, from within the pictorial space (in pointed defiance of Alberti's rules of painting). They elucidate the various ways in which "Caravaggio's figures resist being read only as that which they are meant to represent". As they say of a late painting of St John the Baptist, the model "nearly eliminates the saint from the painting by the force of his own physical personality." This happens time and again with Caravaggio, and it sets up intolerable tensions between a painting's nominal meaning and its actual implications, tensions that the painter was clearly intent on exploring, and that greatly disturbed elements of his audience.
In viewing his work as "a visual speculation on the meaning and conditions of knowledge," they might be accused of projecting their own preoccupations onto a mutely acquiescent body of work, yet, through their application of a mixture of iconographic speculation, and psychoanalytic and literary theory, they provide an illuminating and compelling response to real questions raised by the pictures. For all its virtues, Caravaggio's work is limited in its scope, and perhaps his current popularity will diminish with the next swing of the pendulum, but these various attempts to get to grips with the painter and the paintings tend to suggest that he really was in many ways shockingly modern - and that he still harbours many secrets.
M, by Peter Robb, Bloomsbury, £25 in UK; Caravaggio's Secrets, by Leo Bersani and Ulysse Dutoit, MIT Press £19.95 in UK