to the middle of the 12th century. Gottfried's version, part of the "courtly" branch of the legend, had a huge influence on later German literature. According to his autobiography, Mein Leben, Wagner decided to dramatise the Tristan legend after his friend, Karl Ritter, attempted to do so, writing that: He had, in fact, made a point of giving prominence to the lighter phases of the romance, whereas it was its all-pervading tragedy that impressed me so deeply that I felt convinced it should stand out in bold relief, regardless of minor details. This influence, together with his discovery of the philosophy of Arthur Schopenhauer in October 1854, led Wagner to find himself in a "serious mood created by Schopenhauer, which was trying to find ecstatic expression. It was some such mood that inspired the conception of a Tristan und Isolde." Wagner wrote of his preoccupations with Schopenhauer and Tristan in a letter to Franz Liszt (16 December 1854): Never in my life having enjoyed the true happiness of love I shall erect a memorial to this loveliest of all dreams in which, from the first to the last, love shall, for once, find utter repletion. I have devised in my mind a Tristan und Isolde, the simplest, yet most full-blooded musical conception imaginable, and with the ‘black flag’ that waves at the end I shall cover myself over – to die. By the end of 1854, Wagner had sketched out all three acts of an opera on the Tristan theme, based on Gottfried von Strassburg's telling of the story. While the earliest extant sketches date from December 1856, it was not until August 1857 that Wagner began devoting his attention entirely to the opera, putting aside the composition of Siegfried to do so. On 20 August he began the prose sketch for the opera, and the libretto (or poem, as Wagner preferred to call it) was completed by 18 September. Wagner, at this time, had moved into a cottage built in the grounds of Wesendonck's villa, where, during his work on Tristan und Isolde, he became passionately involved with Mathilde Wesendonck. Whether or not this relationship was platonic remains uncertain. One evening in September of that year, Wagner read the finished poem of "Tristan" to an audience including his wife, Minna, his current muse, Mathilde, and his future mistress (and later wife), Cosima von Bülow. By October 1857, Wagner had