"This picture [ 'Man and Woman contemplating the Moon',] full of sentiment and the quietness of nature, was painted by Friedrich in 1819 and he gave it to me in exchange for one of my own works. Friedrich had to copy it several times, but he did not approve of this, hence others copied it as well. Only the deserved destination for the picture, the Royal Picture Gallery, could convince me to part with it."
January 1, 1970
https://en.wikiquote.org/wiki/Caspar_David_Friedrich