The most comprehensive primary source for the understanding of Van Gogh as an artist is the collection of letters between him and his younger brother, art dealer Theo van Gogh.[7] They lay the foundation for most of what is known about the thoughts and beliefs of the artist.[8][9] Theo provided his brother with both financial and emotional support. Their lifelong friendship, and most of what is known of Van Gogh's thoughts and theories of art, is recorded in the hundreds of letters they exchanged between 1872 and 1890: more than 600 from Vincent to Theo and 40 from Theo to Vincent.
Although many are undated, art historians have generally been able to put them in chronological order. Problems remain, mainly in dating those from Arles, although it is known that during that period, Van Gogh wrote 200 letters to friends in Dutch, French and English.[10] The period when Vincent lived in Paris is the most difficult for historians to analyze because the brothers lived together and had no need to correspond.[11] In addition to letters to and from Theo, other surviving documents include those to Van Rappard, Émile Bernard, Van Gogh's sister Wil and her friend Line Kruysse.[12] The letters were first annotated in 1913 by Theo's widow Johanna van Gogh-Bonger who explained that she published them with "trepidation" because she did not want the drama in the artist's life to overshadow his work. Van Gogh himself was an avid reader of other artists' biographies and expected their lives to be in keeping with the character of their art.