To those who knew him in London, Alexander – Sasha – Piatigorsky, who has died aged 80, cut a shambling, if imperturbable, figure. Few at the School of Oriental and African Studies (Soas), where he was a professor, or in the streets of Lewisham, where he lived for many years, would have guessed that this was a man who was widely considered to be one of the more significant thinkers of the age and Russia's greatest philosopher.
His Symbol and Consciousness: Metaphysical Discussion of Consciousness, Symbolism and Language (1982), written with his friend the philosopher Merab Mamardashvili, is regarded by many as the most complex, and by some as the most important, philosophical work produced in Russian, and it achieved cult status for him. It explores the theory of consciousness, and is a kind of philosophical conversation between the two men, from the respective perspectives of Edmund Husserl's phenomenology and the Buddhist school of Vijnanavad. Written in the two years before Sasha left Russia for Britain in 1974, its manuscript was smuggled out to London by his friend the political philosopher Ernest Gellner.
Sasha joined Soas in 1975 as a lecturer, initially in the history department. Sasha loved Soas; and, to an extent, Soas loved Sasha, although his eccentric dress-style, bohemian manners, thick Russian accent and overt intellectualism bewildered many of his colleagues. But Soas in general, unsurprisingly, did not really know what this Russian, Jewish, Buddhist, philosopher, historian, intellectual, linguist (he knew Sanskrit, Tamil, Pali, Tibetan, German, Russian, French and English) and writer was all about.
Symbol and Consciousness was followed in 1984 by Buddhist Philosophy of Thought, then Mythological Deliberations (1993), Who's Afraid of Freemasons? (1997), Thinking and Observation (2002) and Introduction to the Study of Buddhist Philosophy (2007). His book on freemasonry showed that any movement whose outlook is inseparable from ritual will assume the mental character of a religion. Despite his impressive output, publishing, in his view, was at worst a waste of time and at best a potential impediment to thought. This view, often expressed, did not help his promotion to professor at Soas, ultimately achieved through the direct intervention of Sir Isaiah Berlin when, in 1990, he was appointed professor of the ancient history of south Asia.
Sasha found toeing the academic line somewhat irksome. Once, in the Soas lift, a then director of the school drawled:
"Ah! Piatigorsky, I forgot to tell you ..."
"Stop there, director. Stop! I beg you – stop! As the great Plotinus never tired of saying, forgetting is essential. Any fool can remember ..." and he swept out.
Sasha was born in Moscow, where his father, Moshe, was an engineer and lecturer at the Stalin metallurgical college. When the second world war broke out, Moshe was transferred to the Urals, where he worked as a chief engineer in weapons production, into which activity the young Sasha was also inducted.
Sasha read philosophy at Moscow State University, graduating in 1951. The following year, he and his young architect wife, Marina, moved to Stalingrad, where Marina was engaged in the daunting task of helping to create a new city from the rubble left behind by the Nazis. Sasha worked as a high-school history teacher. One of his students, Tanya, fell in love with him, and they had an affair. She, eight months pregnant, followed him and Marina back to Moscow. Sasha promptly left Marina and married Tanya, but she later left him for another man, and for a while there was a gloomy menage a trois, which involved Sasha sleeping on the kitchen floor.
In 1957 he joined the Moscow Institute of Oriental Studies and, six years later, in Tartu, Estonia, came under the influence of the great semiotician and culturologist Yuri Lotman. He contributed, with others in the so-called Tartu school of semiotics, to the creation of a theoretical framework for the semiotics of culture.
He participated, too, in the Soviet human rights movement. In the wake of the infamous 1965 show trial of the writers Yuli Daniel and Andrei Sinyavsky, Sasha, along with other intellectuals, signed a letter deploring the violation of the writers' human rights, and later took part in the first human rights demonstration in Pushkin Square. The year before, Sasha's friend the Russian-Jewish Nobel prize-winning poet Joseph Brodsky had been sentenced to five years' internal exile.
Sasha, as a Jew, Buddhist and non-political intellectual cult figure, was no doubt under KGB surveillance. He and his Indologist friends would meet up in a room at the institute, which they all assumed to be bugged, so fiery debates would be conducted in Sanskrit. He was expelled from his post in 1968, the same year he married his third wife, Elya.
Sasha never had anything formally to do with the Jewish community, either in Moscow or later in London, although he constantly invoked his Jewish heritage with the phrase "I am a Jew, a very bad Jew." It enabled him nevertheless to get a one-way ticket to leave the Soviet Union in 1974, at a time when Jewish intellectuals were being allowed to leave for Israel. However, instead the family proceeded to Oxford, where initially Sasha had been invited to give some lectures. There, he struck up friendships with Isaiah Berlin and Leszek Kolakowski. He moved to Soas the following year.
I bumped into Sasha in central London shortly before he took retirement from Soas in 2001. He was accompanied by a young woman clasping a huge white teddy bear, a present from Sasha. This was Liudmila, his fourth wife. They set up home near King's Cross, with little money, and with her devoted support he happily threw himself into this new life.
During the 1980s, Sasha had spoken regularly on the Russian service of the BBC, generally on philosophical matters, and achieved a considerable following. As a result, when his first philosophical novel, The Philosophy of One Street, was published in Moscow in 1994, it created a stir. It was followed by Remember the Strange Person (1999) and An Ancient Man in the City (2001).
With the collapse not only of the Soviet system in 1991, but of its philosophical underpinning, Russians were left in a philosophical void. Sasha contributed to filling the vacuum. For the last dozen or so years of his life, he became an intellectual superstar in Russia. His advice was sought by powerful politicians seeking non-Marxist inspiration, and he was regularly interviewed on television and radio. Over the past three years, he held a monthly seminar in London to which people would come from Russia, some flying in by private jet.
His last philosophical books and articles in Russian dealt with political philosophy. For some years he had taken issue with Berlin's view, which saw the USSR in terms of the conflict between innocent victim and ruthless totalitarian regime, by highlighting the complicity of Russian intellectuals in fostering and even creating the regime. Sasha was highly critical of many Russian dissidents, arguing that they were just as likely as conformist apparatchiks to be stuck in outmoded ideological frameworks.
His excoriating attacks on Russian intellectual life were in a sense in the tradition of Old Testament prophecy. Although it is difficult to define Sasha's philosophy, which was in a state of constant flux, his Thinking and Observation (2002) provided a short summation of his recent thought. He suggested here that human consciousness depends on the contemplation of material or mental objects, not on the objects themselves. Consciousness is concerned with how we observe. It thinks about thinking. His rejection of what he considered "cardboard reality" drew on Buddhist thought.
His most recent symposiums had attempted to elaborate what he called a "non-anthropocentric philosophy". This new direction came to him amid the chaos which afflicted his life before he found his council flat. Surrounded by packing cases, papers and mess, he began ruefully to conclude that human beings were considerably less important than they have the habit of assuming.
He is survived by Liudmila, three daughters and two sons.
• Alexander Moiseyevich Piatigorsky, philosopher and writer, born 30 January 1929; died 25 October 2009
• This article was amended on Monday 18 January 2010. Joseph Brodsky was sentenced to internal exile in 1964, not 1965 as we suggested. This has been corrected.