Dr. Oakes, is it possible that anonymous prophecies became part of canon?
The fact is that we do not know the provenance (history and source) of all of the Old Testament scripture. I believe that Andrew Davidson is rather grossly exaggerating the extent to which editors put together materials, but the fact is that we cannot be absolutely sure that everything which is found in Isaiah was in fact written by Isaiah. It is tempting for believers to assume that there is no truth in the claims of Davidson, but if we have too simplistic an approach to understanding the Old Testament scriptures, we may find ourselves struggling when we are presented with evidence that bring into doubt our simplistic assumptions.
Let me give a couple of examples.
It has been simplistically (in my opinion) assumed that Moses either wrote or compiled the entire Pentateuch. Jews have made such claims and many Christians have simply accepted this premise. Yet, there is good reason to believe that Moses had little if any role in putting the book of Genesis together, and he certainly did not write the part of Deuteronomy which records his death. The problem is that when we make this simplistic assumption, unbelievers will not respect our arguments and will be more tempted to dismiss our belief in the inspiration of the Bible.
Psalms is another example. Some of the Psalms are credited to David, but it is fairly likely that some of those credited to David were written in the spirit of David, but not actually by David. Whether any particular claim about the authorship of a particular psalm is true or not is not the point. Simplistic assumptions are not a good approach for mature Christians to take with regard to the Old Testament text. Intelligent, well-educated, open-minded people will not take us seriously if we make simplistic assumptions.
A third example is the book of Isaiah. A significant majority of scholars believe that Isaiah did not write chapter 40-66. Personally, I am skeptical of this claim. In fact, I believe that this conclusion is partially based on presuppositions which reject the inspiration of the Bible! I believe that Isaiah wrote all of Isaiah, but I am not certain about this. Besides, these scholars do have reasonable points to make about the change in style and content of the last 27 chapters. It is possible that they are right. My point is this. We ought to listen to the critics and not simply blow off their proposals. If we do we run the risk of being wrong, and of losing our ability to influence skeptics.
Is there material contained in the prophets which is associated with a particular prophet, but which was not actually written by that prophet (your question)? My answer is that I do not know. It is entirely possible that some prophecies were considered inspired by Jewish scribes and that they included them with material of known authorship. I believe that it is a mistake to outright dismiss such claims without listening to the arguments of those who make these claims. Like I already said, I believe that Davidson is GROSSLY exaggerating here. I believe that Ezekiel wrote all or at least nearly all of the book of Ezekiel. There is no way that this amazing book is the work of many anonymous authors! The same with Isaiah, Hosea, Jeremiah and the other prophets. But… Is it possible that some anonymous material was included in some of the prophets? Yes, it is possible.