After he wrote his first 15 sonatas, he wrote to Wenzel Krumpholz, "From now on, I'm going to take a new path." Beethoven's sonatas from this period are very different from his earlier ones. His experimentation in modifications to the common sonata form of Haydn and Mozart became more daring, as did the depth of expression. Most Romantic period sonatas were highly influenced by those of Beethoven. After his 20th sonata, published in 1805, Beethoven ceased to publish sonatas in sets and published all his subsequent sonatas each as a single whole opus. It is unclear why he did so.