Future historians trying to account for the institutionalised fraud that goes under the name of "Theory" will surely accord a central place to the influence of the French psychoanalyst Jacques Lacan. He is one of the fattest spiders at the heart of the web of muddled not-quite-thinkable-thoughts and evidence-free assertions of limitless scope, which practitioners of theorrhoea have woven into their version of the humanities. Much of the dogma central to contemporary theory came from him: that the signifier dominates over the signified; that the world of words creates the world of things; that the "I" is a fiction based upon an Oedipalised negotiation of the transition from mirror to symbolic stages; and so on. The English translation of this biography by one of his disciples is therefore an event of the first importance. It is a harrowing read, but no one who inflicts on students Lacanian readings of literature, of feminism, of the self, of child development, of society, or of life, should be spared the experience.

Lacan was born in 1901 into a wealthy middle-class family and trained as a doctor. He was attracted first to neurology but soon abandoned this because the patients' troubles were too "routine", as his biographer (who clearly sympathises with his inhumanity) explains. If Elizabeth Roudinesco's account is accurate, he must have made a hash of his first case presentation to the Societe Neurologique: his patient, she says, supposedly had "pseudobulbar disorders of the spinal cord" - a neurological impossibility. (The innocence with which Roudinesco reports all kinds of clinical cock-ups makes this book a particularly disturbing read for a medic.) Abandoning neurology was obviously a wise career move. Unfortunately, though he lacked all the qualities necessary to make a half-way decent doctor (eg kindness, common sense, humility, clinical acumen and solid knowledge), Lacan did not abandon medicine altogether, only its scientific basis. He chose to be a psychoanalyst where, instead of elucidating diagnoses, he could impose them.

He fastened on Marguerite Pantaine, a tragically deluded woman who had attempted to kill a well-known actress. For a year, he and Marguerite were, according to Roudinesco, "inseparable". (She had no choice, being in detention.) The elaborate story he concocted about her became the basis of an entire theory of the sick soul and formed his doctoral thesis. In the great tradition of psychoanalysis, "he listened", Roudinesco says, "to no truths other than those which confirmed his own hypotheses". More precisely, the truth was that which confirmed his hypothesis: into her case, "he projected not only his own theories on madness in women but also his own fantasies and family obsessions". For this soul-rape Lacan was awarded his doctorate and his reputation was made. To the end of her days, Marguerite remained bitterly resentful of the use he had made of her. With good reason: Lacan's crackpot theories, partly expropriated from Salvador Dali, probably prolonged her incarceration. To add insult to injury, he "borrowed" all her writings and photographs and refused to give any of them back.

Lacan published few further cases of his own. Instead, he recycled some of Freud's well-known cases, in pursuit of his avowed aim of restoring the truth of Freud's ideas which he believed had been traduced by Freudians. Unfettered by data, he was free to soar and to promulgate those large, untestable and obscure ideas - they were too difficult even for Melanie Klein to understand - that made him into an international superstar and which were cherished by his followers and are foundational for theorrhoeists. His doctrines - a magpie muddle of often unacknowledged expropriations from writers whose disciplines were alien to him, cast in borrowed jargon and opaque neologisms -were Rorschach ink-blots into which anything could be read. Lacan's ideas were insulated against critical evaluation by his writing style, in which, according to Roudinesco, "a dialectic between presence and absence alternated with a logic of space and motion".

The most powerful support for his doctrines, however, was the aura which surrounded him. Lacan was a handsome dandy and, like many physically attractive psychopaths, he was able to command unconditional love. He exploited this to the limit in support of his boundless appetite for wealth, fame and sex. He kept his disciples, who "worshipped him like a god and treated his teachings like a holy writ", in constant fear of excommunication: the absence of Lacan was an ontological catastrophe equal to the absence of God. Anyone who fell under the spell of the Master laid aside their critical sense.

He justified his intellectual terrorism on the grounds that he was surrounded by enemies whom he had to fight. One lot of enemies he conspicuously did not fight were the occupying German forces during the second world war. Although he remained in France, he so ordered his affairs as to be entirely safe and entirely comfortable. He felt, according to an admirer, Jean Bernier, that "the events that history forced him to confront should have no effect on his way of life, as befits a superior mind". As a doctor he had many privileges and he made full use of them. The major battles of his life were therefore in peace-time, most notably with the International Psychoanalytical Association (IPA) from which he was eventually expelled in 1963.

Lacan portrayed this break as the result of an ideological conflict between the old school and the progressive, true Freudians represented by himself. Actually it was about his greed. He needed to maximise his throughput of patients in order to finance his lavish lifestyle. (He died a multi-millionaire.) He started to shorten his sessions, without a pro rata reduction of fee, to as little as ten minutes. Unfortunately, Freudian theory fixes the minimum length of a session at 50 minutes. Lacan was therefore repeatedly cautioned by the IPA. According to Roudinesco, he gave several lectures to the Societe Psychanalytique de Paris arguing that shorter sessions produced a beneficial sense of frustration and separation in the patient, "turning the transference relationship into a dialectic" and "reactivating unconscious desire". Additionally, he lied to the IPA about the duration of his sessions. Despite this belt-and-braces approach, he was rumbled and out he went.

Faced with loss of income, he established his own French School of Psychoanalysis, over which he had absolute power. Its work, Roudinesco says, "concentrated on desire, transference and love, and all of these came to be focused on the person of Lacan himself". Now he could make his sessions as short, and as expensive, as he liked. Even when they had contracted to a minute or two, he would often see his tailor, his pedicurist and his barber while conducting his analyses. In the final years, the process of shortening reached its natural conclusion in the "non-session", in which "the patient was not allowed either to speak or not to speak" as Lacan "had no time to waste on silence". With the help of non-sessions he averaged 80 patients a day in the penultimate year of his life. Non-sessions were perhaps an improvement on sessions, in which, disinhibited through dementia, he would indulge his bad temper, raging at patients and occasionally punching them or pulling their hair.

The calamitous consequences of his style of doctoring were entirely predictable: his clients committed suicide at a rate that would have alarmed a man armed with less robust self-confidence. He claimed that it was due to the severity of the cases he took on but it may also have had something to do with the way he would start and stop analysis at whim and would sometimes cast aside, at very short notice, people who had been under his "care" for years. The brilliant ethnologist Lucien Sebag killed himself at 32 after having been been discharged abruptly from treatment - because Lacan wanted to sleep with Sebag's teenage daughter. Not that Dr Lacan was always so constrained by such exquisite moral scruples. He frequently chose his mistresses from his training analysts (who were additionally vulnerable because they relied on him for the pass necessary for them to practise as Lacanian analysts) and also from his ordinary analysands. In his defence, Roudinesco points out that Lacan never pursued the physical side of things in his consulting rooms. One suspects that, given the design of the analyst's couch, this was dictated by mechanical rather than ethical constraints.

On the principle of credo ut intelligam his disciples still believed him even when, in his last few years, he was manifestly suffering from multi-infarct dementia. He became obsessed by a particular mathematical figure called a Borromean knot, in which he saw the key to the unconscious, to sexuality and to the ontological situation of mankind. His quasi-mathematical, pseudo-logical fantasies - the culmination of the cargo cult science of his school - propounded in interminable seminars, were agonised over by his congregation who suffered appallingly from their inability to make sense of them. They felt unworthy of the Master. Even his episodes of aphasia, due to mini-strokes, were taken to be "interpretations", in the technical sense of conveying "the latent meaning of what the analysand has said and done". When, towards the end, he became deaf and his responses were even more disconnected from what was said to him, this occasioned protracted arguments among his followers over the meaning of his words and deeds. Even when, in his last year, Lacan's mind was entirely vacant, he was still brought to meetings "to legitimise what was being done in his name" and "suggestible people heard him speak through his silence".

When he died in 1981, total war broke out among his disciples. Within a decade, there were 34 associations claiming to be the sole representative of the true spirit of Jacques Lacan and the sole heirs to his intellectual estate. Even now, 15 years after his death, this extraordinary charlatan can still command the adoration of the vulnerable and the gullible. Roudinesco, for all that she dishes enough dirt to hang Lacan ten times over, seems to forgive him everything for his "genius" as a clinician and thinker. Nor does she question any of his fundamental ideas, though in the course of a 500-page book she disdains either to expound them in any coherent way or to offer any evidence for them: she is too busy with splits, schisms and influences. It is apparently enough proof of their truth that Lacan asserted the doctrines associated with his name.

His lunatic legacy also lives on in places remote from those in which he damaged his patients, colleagues, mistresses, wives, children, publishers, editors, and opponents - in departments of literature whose inmates are even now trying to, or pretending to, make sense of his utterly unfounded, gnomic teachings and inflicting them on baffled students. Aleister Crowley, the 20th-century thinker whom Lacan most resembles, has not been so fortunate in his afterlife.

Lacanians may argue that the great edifice of the Ecrits is not undermined by revelations about his life: the Master's thoughts should be judged on their own merits. However, in the absence of any logical basis or empirical evidence, the authority of the thought has derived almost completely from the authority of the man. The discovery that Lacan was the shrink from hell is not, therefore, irrelevant. Roudinesco's biography is consequently an act of liberation on behalf of those students, forced by uncritical teachers who do not know Stork from butter, to try to understand and make sense of his nonsense. This act of liberation is all the more compelling for being the work of a disciple and thus in part involuntary.

Raymond Tallis is professor of geriatric medicine, University of Manchester.