INTRODUCTION

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      In July 1764 a striking young woman was sitting at the window of a remote country château in Holland, carefully writing a secret letter. The château – with fairytale turrets, a deep moat, and lines of poplar trees stretching to a distant view of canals and windmills – was her family home: the seventeenth-century castle Zuylen, hidden away deep in the peaceful farmlands near Utrecht.

      The young woman was Isabelle de Tuyll, the eldest daughter of the aristocratic de Tuyl de Serooskerken family, governors of Utrecht. The secret letter she was writing was addressed to her new-found confidante, a glamorous Swiss aristocrat and career soldier, the Chevalier Constant d’Hermenches. Isabelle was twenty-four years old, headstrong, and unattached. The Chevalier d’Hermenches was married, almost twice her age, and had the reputation of a libertine. He wore a dashing black silk band around his temples, to hide a war wound. Their clandestine correspondence was being smuggled in and out of castle Zuylen via a compliant Utrecht bookseller.

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      If it was an unusual situation, then Isabelle de Tuyll (known by the local landowners as ‘Belle de Zuylen’) was a thoroughly unusual young woman. If not exactly beautiful, she was extremely attractive and drew glances wherever she went. She had a full, open face, with large green eyes and wild auburn haired brushed impatiently back from a high forehead. She was tall, commanding, full-bosomed, and restless in all her movements. She was not only an accomplished harpsichord player (and composed her own music), but also an expert shuttlecock player, quick and determined in her strokes, with an almost masculine speed and self-confidence.

      Isabelle de Tuyll was also unusually well-educated. Her liberal-minded father, recognizing the exceptional talents of his eldest daughter, had spared no expense. As a girl she was given a clever young governess from Paris, Mlle Prevost, and she was soon studying French, Latin, Greek, mathematics, music, algebra, and astronomy. Later she was tutored by a mathematics professor from Utrecht university. She took a particular delight in reading Voltaire and calculating conic sections. She once remarked nonchalantly: ‘I find an hour or two of mathematics freshens my mind and lightens my heart.’

      By the time she was twenty in 1760, Isabelle de Tuyll was being besieged by suitors, proving particularly attractive to rich Dutch merchant bankers and penniless Germain princelings. But now she was too quick and clever for most of them. They bored or irritated her. The first time she ever saw the Chevalier d’Hermenches, at the Duke of Brunswick ball in the Hague, typically she broke all the rules of etiquette by going straight up to him and asking ‘Sir, why aren’t you dancing?’ At first he was deeply offended, then quickly charmed. ‘At our first word, we quarreled,’ he said later, ‘at our second, we became friends for life.’ It was soon after that their clandestine correspondence began.

      Though admired by her younger siblings (and adored by her brother Ditie), her parents now thought of Isabelle with increasing anxiety. In the salons of Utrecht and the Hague, she was getting the reputation of a belle esprit, an unconventional free spirit, a rationalist, a religious sceptic: in short, a young person of ‘ungoverned vivacity’. This was all very well for a man, but perilous for a young woman. She might never marry and settle down. Her name ‘Belle de Zuylen’ was now spoken with a certain frisson.

      To confirm these worries, Isabelle soon began to write poems, stories