Who said 'I would like to show by my work what this nobody has in his heart'?

This is a partial quote from Vincent Van Gogh in a letter to his brother Theo, dated 21 July, 1882. The full quote from the letter is: "All right, then - even if that were absolutely true, then I should one day like to show by my work what such an eccentric, such a nobody, has in his heart." Vincent wrote many letters to his brother Theo, which have been preserved, and they give us quite an insight into what the artist was thinking and feeling.