He believed that what gives arts such as literature and sculpture their value was the extent to which they incorporated pure perceptions. But, being concerned with human forms (at least in Schopenhauer's day) and human emotions, these art forms were inferior to music, which being a direct manifestation of will, was to Schopenhauer's mind the highest form of art. Schopenhauer's philosophy of music was influential in the works of Richard Wagner. Wagner was an enthusiastic reader of Schopenhauer, and recommended the reading of Schopenhauer to his friends. His published works on music theory changed over time, and became more aligned with Schopenhauer's thought, over the course of his life. Schopenhauer had stated that music was more important than libretto in opera. Music is, according to Schopenhauer, an immediate expression of will, the basic reality of the experienced world. Libretto is merely a linguistic representation of transient phenomena. Wagner emphasized music over libretto in his later works after reading Schopenhauer's aesthetic doctrine.