Van Gogh's early biographer, C. Lewis Hind, wrote the artist's letters are "soliloquies which Van Gogh held in the presence of his own soul." He added, "The communications to his brother and his friend," [fellow artist, Emile Bernard, who van Gogh wrote to from 1887 - 1890] are "a running commentary to his lifework, a Sabbath's meditation upon and contemplation of his six days' labour..."

 

As you'll see, in his letters, van Gogh sometimes reverts to his years as an art dealer as he assesses the quality and "commercial value" of his own work.