Why is Isaiah 40-55 considered to be the work of someone other than the prophet Isaiah?

A:There are differences in historical interest. The author of most of the material in Isaiah 1-39 (known today as FirstIsaiah) is concerned with Assyria as the dominant power, the kings with whom he associated are those of the last half of the eighth century, and the setting of his work is this period. The Babylonian Exile is the background forchapters 40-55 and the author (now known as Second Isaiah)

is concerned with interpreting the Exilic experience. The dominant nation is Babylon, not Assyria. Interest is focused on a new power, Persia, and Cyrus is even named and recognised as a deliverer of the people (Isaiah 45:1), facts that First Isaiah could not have known about or even understood because the

Persian nation did not come into existence until after his time

.

Isaiah 40-55 form a unity of thought and emphasis, withstriking stylistic variations and differences in vocabulary compared to the earlier work.