Correct me if I am wrong, but doctrine cannot be unless it is restated and corroborated from other perspectives in multiple sources. Alone, the book of Psalms states truths, but in conjunction with the entire Scriptures, the book of Psalms states doctrine. Alone, the book of Psalms is sufficient to be true, yet (I would argue) are not efficient. ex. In chapter 22, there is a truthful prophesy of Christ’s death. However, viewing this prophesy in conjunction with all the gospels/ epistles, the entire picture of Christ’s death, which is corroborated over and over, becomes doctrine. So now, Psalms 22 does not only prophesy of Christ’s death, but preaches the doctrine of His death. When I sing psalms, I do not sing it from the point-of-view of Psalms only, but from the point-of-view of the entire scriptures (which the book of Psalms fully corroborates. ie. The full collection of Psalms does not contradict any other portion of the Bible.)