The Book of Psalms is made up of 150 songs or prayers of various lengths. Each is a poem with frequent use of parallelism and other poetic techniques. According to tradition, many of the Psalms were composed by the Israelite King David. Written by the fourth-century theologian and philosopher Augustine of Hippo, Expositions on the Book of Psalms is the first part of a six-volume work of commentary on the Old Testament book. Augustine was a Latin philosopher and theologian generally considered one of the greatest Christian thinkers of all time.