Dutch-born painter, graphic artist and sculptor of Scottish descent who spent most of his life in Germany. Because he attended school and exhibited his pictures in Wiesbaden, and had the Freudenberg Castle where he was to reside built on the edge of town, he is counted among the Wiesbaden group of painters.
From 1883 Knowles studied at the Academy of Fine Arts in Munich under Carl Theodor von Piloty and Fritz von Uhde. He then attended the Weimar Art School under Leopold von Kalckreuth. All three teachers are likely to have influenced Knowles’ realistic painting style, which he usually practised until the 1950s. In 1887, Knowles travelled to Paris to continue his education at the Académie Julian in the studio of Jean-Paul Laurens.
He befriended the Hungarian painter József Rippl-Ronai and around 1888 Knowles and Ronai came into contact with Les Nabis, in whose community both were accepted. Les Nabis were a group of young French artists active in Paris from 1888 until 1900, who played a large part in the transition from impressionism and academic art to abstract art, symbolism and the other early movements of modernism.
In Paris, Knowles began a relationship with ‘Yvonne,’ born Marie-Eugénie-Guérinet Victoria (1870-1959), a daughter of Napoleon III and a lady-in-waiting, whose godmother was Napoleon III’s wife, Empress Eugénie.
He returned to Wiesbaden in 1891 but typhoid drained his strength and he did not paint for three years. In January 1895 he exhibited together with August von Heyden and Hubert von Heyden in the Salon of Fritz Gurlitt.
From 1902, he led negotiations for the purchase of a site on a hill in Wiesbaden-Dotzheim overlooking the Rhine. First, Swedish carpenters built a solid log house which was completed in 1903. Knowles and Yvonne/ Marie-Eugénie-Guérinet Victoria commissioned the building of a castle, completed in autumn 1905 and named Schloss Freudenberg. The couple hosted generous parties and balls, and after they separated in 1909, the castle was sold.
In 1919, Knowles married thirty-three-year-old Louise, Princess of Solms-Braunfels (1885–1964). The couple spent the following years in the Solms Villa in Bad Homburg, and the Solms castles in Lich and Braunfels. Knowles spent the last decades of his life with his wife in Hungen Castle in the district of Gießen.
When England declared war on Nazi Germany on 3 September 1939, Knowles applied for German citizenship but this was rejected and he was placed under house arrest, with his wife.
Their confinement in the Hungener Schloss was limited to a few rooms, while the Nazi henchman Alfred Rosenberg took over the majority of the castle which was set up as a depot for art looted from Jewish citizens.
When the 3rd U.S. Armored Division appeared in the nearby forest of Hungen in early January 1945, the eighty-two-year-old Knowles ran across the fields towards the U.S. Armed Forces, with a white peace flag in one hand and the Union Jack waving over his head with the other. No shot was fired after Knowles informed the GIs that the German military and Nazi officials who managed the confiscated Jewish objects had fled Hungen. The Hungen castle thus escaped a “bombing” at the end of the Second World War due to the Scottish painter.
Pitcairn-Knowles and his wife continued to live in the Hungener Schloss after the war, which, like other castles, soon filled with refugees. When Knowles died at age 90 on 2 January 1954, seventy refugees still lived in the castle.

Link to Wikipedia biography (German)