"For the first time, I did not care if the result would be to the good taste of the public," said writer-director Ingmar Bergman of the boldly experimental "Persona". Made in the mid-60s, his film's intensity and mysteriousness remain undimmed despite the passage of time.
Ostensibly it charts the fraught relationship between two women, who are staying at a cottage on the remote island of Faro. Elisabet (Liv Ullmann) is an actress, who appears to have lost the power of speech ever since she froze on stage playing Electra and was sent to a psychiatric clinic.
Alma (Bibi Andersson) is her cheerful nurse, who fills the time by talking about her boyfriend and her past sexual encounters.
Opening one of Elisabet's letters however, Alma discovers that Elisabet actually regards her as an amusing object to study, prompting a hysterical counter-reaction on the carer's part...
Bergman believed that "film should communicate psychic states, not merely project pictures". It's therefore appropriate that "Persona", with its doublings, mirror imagery, and uncanny fusions, should so seamlessly merge reality and fantasy.
This makes it hard to say what definitively happens to its characters, as opposed to what they might have imagined.
Strikingly self-referential - the opening credits include shots of a film projector, whilst during one argument the film itself breaks down, melting a hole in the screen - "Persona" lends itself to a variety of plausible interpretations.
Partly it can be seen as a metaphor for the process of psychoanalysis, with Elisabet as the silent analyst and Alma the regressing patient, whose desires eventually erupt.
Partly it explores the potentially exploitative relationship between artist and audience, and the difficulty of art in responding to the horrors of "real" life: thus Elisabet is visibly shocked by TV images from Vietnam and World War Two.
And perhaps most fundamentally, "Persona" examines, to quote the film's female psychiatrist, "the hopeless dream to be", and how hard it is to penetrate beyond the masks we wear and the roles we play in our lives.
Brilliantly performed by Andersson and Ullmann, and atmospherically photographed by Sven Nykvist, this is one of the landmark films of post-war cinema.