In 1688, Witsen was visited and invited more than once to discuss
William III's proposed crossing to England, but he had great doubts and did not know what to advise.
William Bentinck called him the most sensitive man in the world. Of the other three burgomasters – Jean Appelman (a merchant trading with France),
Johannes Hudde and
Cornelis Geelvinck – Geelvinck openly opposed the enterprise and Appelman was not trusted by the prince and thus was not informed. After the crossing went ahead, Witsen went to London in the next year to find a way of meeting the costs of 7,301,322 guilders the city of Amsterdam had incurred in supporting it. William offered to knight him as a
baronet, but the modest Witsen refused. Witsen, who had invested in the journey, was disappointed that the men had been more interested in setting up trade than in exploring. Witsen tried to introduce
coffee plants from
Batavia via Amsterdam to countries in South America. Witsen had contact with the painter
Jan de Bray over a plan to improve the city's water supply, and helped the artist
Cornelis de Bruijn, who needed contacts in Egypt and Russia – indeed, it was probably Witsen who encouraged De Bruijn to make drawings of
Persepolis, to show to the
Royal Society. He helped
Maria Sybilla Merian to publish her prints with plants and insects from
Surinam. Witsen was also interested in religion but in an
ecumenical way: his interests stretched to "saint
Confucius" as he called him (based on his analysis of a Han dynasty Chinese mirror in his collection), as well as to
shamanism. The minister famous for attacking witch-hunts,
Balthasar Bekker, was his friend. == Death ==