Documentaries document. Or so one might suppose, given the opprobrium recently heaped on Michael Moore for the 'undocumentary' imbalances and excesses of his Fahrenheit 9/11. Had the attacks come from the Bush-supporting right then they could have been dismissed as a simple political response to a simple political film. But it is a section of the liberal intelligentsia that has turned against Moore, including some of those who were so enthusiastic about his earlier attacks on American gun culture in Bowling for Columbine (2002) and General Motors in Roger & Me (1989). Aside from distaste at his undoubted penchant for self-publicity, it's an odd response. The idea that non-fiction films should be balanced reflections of reality – BBC current affairs strictures notwithstanding – is deeply misguided, and in any case never really survived the first few years of the documentary movement. John Grierson, who was hugely influential on the shape of documentary from the late 1920s onward, believed in fostering a 'purposive cinema', one which would play a positive role in sustaining and extending democracy. By informing and educating the public, he (and many of the film-makers whose careers he encouraged) hoped to promote an essentially reformist agenda. In making films like Industrial Britain (1932) or Housing Problems (1935), though they were indeed 'documenting' social states of affairs, they also knew very well that they were intervening in reality and not simply reflecting it.

There have been cinema verité moments, of course, when documentarists have laid claim to privileged access to the world recorded by their cameras. But more often than not, that was either self-delusion or a cinematic illusion created by skilful manipulation, editing and even staging. Ever since Flaherty deliberately set up Nanook and his fishing hole, documentarists have been persuading reality to fit their projects. The direct cinema film-makers of the 1960s – the Maysles brothers, Leacock, Pennebaker – may have used the new lightweight sound and camera technologies to go where film-makers had previously been unable to go with the avowed intention of capturing the richness of that real world. But they too were not averse to encouraging reality to behave itself when necessary.

Frederick Wiseman, the most thoughtful exponent of the direct cinema philosophy and, for many, the finest living film documentarist, puts it bluntly. "Everything about a movie is manipulation … If you like it, it's an interpretation. If you don't like it, it's a lie – but everything about the movies is a distortion." In recognition of that view, he calls his films 'reality-fictions' rather than documentaries, although they do indeed document the workings and failures of a whole variety of real American institutions. Shooting with a mobile two-person unit, and in his earlier films in black and white, he creates a powerful sense of the ways in which our institutions bear down upon us. Never resorting to voice-over guidance and using only the natural sounds encountered while shooting, Wiseman does much of his work in the editing, moulding his material into extraordinarily compelling and subtle accounts of the worlds which he observes. Welfare (1975) is perhaps his finest film, its three hour length steadily drawing us into the desperate travails of those caught up in the machinery of an overworked welfare agency. But, as the recent, even longer Domestic Violence (2002) suggests, his moral power and film-making skills are still as effective as ever, his style still as persuasively understated.

It would be hard to think of a film-maker further removed from Michael Moore's rabble-rousing sensibilities. Where Wiseman coaxes his audience into thought, providing the time, occasion and inclination for us to reflect, Moore tries to beat us into submission with a polemicist's hammer. Wiseman's style is cool and analytical, though its cumulative effect is often intensely moving, while Moore (as he does with the distraught mother in Fahrenheit 9/11) ladles on the emotion only finally to leave us alienated by that very excess. Where Wiseman clearly has a penetrating understanding of the unintended social consequences of human institutions, Moore is inclined toward escalating and often inconsistent conspiracy theories. And while Wiseman is present artistically in every frame of his films, through his style and his ideas, Moore's presence is crudely literal – as a sometimes disingenuous, sometimes knowing, and sometimes self-serving participant in his own creation.

So, while you may very well sympathise, as I do, with the political tenor of Bowling for Columbine or Fahrenheit 9/11, it is getting increasingly difficult to defend a film-maker so bent on overstating every case that he makes. Not, however, because he is betraying the cause of proper documentary – the documentary movement always has room for fierce commitment, as the exceptional Brazilian film Bus 174 (2002) recently demonstrated – but because of the sheer incoherence of his scattergun approach. With political friends like this, who needs enemies?