There were many Freuds: the scholar, the academic, the researcher, the neurologist, the founder of the new discipline and psychoanalysis, and the Viennese professional. All were noted for their rejection of religion and their identification with prevalent German culture. This was the picture painted by Freud's principal biographers. They all agreed that Freud came from an assimilated Jewish background and he was a completely secular intellectual.

However, more recent studies show a very different and more complicated Freud. This Freud emerged from a deeply religious Chasidic background, with generations of distinguished rabbis and scholars on both his maternal, paternal and marital sides. They show that Freud was very knowledgeable about Jewish ideas and practices and that he was very familiar with both Hebrew and Yiddish.

It is clear that Freud was a master of dissimulation. This Freud was extremely ambitious. He denied what he knew in order to be seen and treated as an eminent German doctor. He was also determined to deflect the pervasive antisemitism in Vienna away from himself and his creation. We can say that there was a revealed or overt Freud and a concealed or covert Freud. The former has been well documented.

I am interested in the hidden or covert dimension of Freud's persona and explore how it reflected his struggle with his Chasidic and Kabbalistic antecedents. My fascination with Freud and the Rebbe does not just refer to his encounters with the Rashab, the fifth Lubavitcher Rebbe, who came to him for help in 1903, but also to disputations with his rabbinic alter ego.

This is not to say that Freud was or wanted to be a believing Jew, far from it. The overt Freud was closer to the genus Judaeus Psychologicus - "psychological Jew" - a term the renowned historian Yosef Yerushalmi coined to denote a man who was charmingly ignorant of Jewish culture and customs. Yet, as Yerushalmi also emphasised, Freud maintained some "strikingly mutant traits".

The covert Freud was proud of and strongly asserted his ancestral lineage. He willingly joined the board of governors of the Hebrew University. And he vehemently objected when a friend did not refer to his beloved chow dog by her correct Hebrew name. Indeed, his writings are littered with Hebrew and Yiddish words and phrases.

Why did Freud choose to denude his work of religious content? Certainly, as an adult, he needed to establish his own identity, in the face of strong familial pressures to follow in the footsteps of his father's father, a pious man, after whom he was named. Hence we see Freud the rebel, who tried to reject his Chasidic heritage. But, his rejection of religion was closely connected to a deeply felt sense of maternal deprivation.

Sigmund was born to Amalie Nathansohn, a new bride and his father's third wife, just past her teens. At the time the Freud household was still mourning the death of his grandfather, Shlomo. Amalie became pregnant again with her second son, Julius. This baby also carried the same name of Amalie's brother and beloved companion.

Baby Julius died from an infection when Sigmund was two years old, and Amalie's brother passed on at around the same time from tuberculosis. Both losses left his mother heartbroken and emotionally unavailable to Sigmund. He was then passed on to a Czech Catholic nursemaid, Resi Wittek, to whom he became very attached, emotionally and physically.

Suddenly, at the age of three, while his mother was confined with his sister, Anna, Resi disappeared, never to return. Sigmund's religious, older stepbrother, Phillip (who later moved to Manchester), found that she had stolen some household money and had her sent to prison.

Both his mother and nursemaid were very observant. It is likely that Freud transferred the sense of rejection and rage he felt for being abandoned into disdain and contempt for Jewish religious observance and ritual, and subsequently, all religious practice. These views reflect the overt Freud. They seem to have surfaced in his grandchildren and their descendants, none of whom have maintained an attachment to Judaism.

The covert Freud was more dutiful. He visited his aged mother every Sunday, even though he suffered from headaches or gastric upset beforehand. And he did try to grapple with his Jewish inheritance through the questions he posed in his last book, Moses and Monotheism.

Freud's lasting legacy is enormous. His creation, psychoanalysis, is a method, a way of thinking, a discipline through which we can find meaning in the experiences which make us human. It is a science of subjectivity enhanced by the process of "free association", which 13th-century Kabbalists called "skipping and jumping". Nowadays practitioners call this "listening with the third ear". It is a listening which is very attentive, non-judgmental and highly sensitive to nuances of thought and feeling.

Psychoanalysis itself carries the added cachet of opening a door to the many discoveries and mysteries of Kabbalah. It is the means by which the Jewish mystical tradition has entered and enriched the mainstream of society. Kabbalistic ideas include the concept of bisexuality, methods of dream interpretation, the interplay between good and evil, theories of repression and depression ("lowness of spirit") and the significance of reparation, tikkun, perhaps the most important imprint of them all.

The Hidden Freud: His Hassidic Roots, by Joseph Berke, Karnac Books, £25.99