Albert is known for his commentary on the musical practice of his times. Most of his written musical observations are found in his commentary on Aristotle's Poetics . He rejected the idea of " music of the spheres " as ridiculous: movement of astronomical bodies, he supposed, is incapable of generating sound. He wrote extensively on proportions in music, and on the three different subjective levels on which plainchant could work on the human soul: purging of the impure; illumination leading to contemplation; and nourishing perfection through contemplation. Of particular interest to 20th-century music theorists is the attention he paid to silence as an integral part of music.