I understand that the book of Daniel is comprised of two languages: Aramaic from chapter 2:4 to 7:25, the narrative part, and Hebrew in chapters 8 to 12, the prophetic and apocalyptic part. I understand also that the Aramaic is debated as to the age but is generally dated around the 4th century (though some say earlier) and the Hebrew is classed as post exilic Hebrew.
I’ve read that some scholars theorise that Daniel was written in two periods: first in Aramaic around 4th century or earlier and the Hebrew section, chapters 8 to 12, was added on around the Maccabean period in the second century. I am aware that there are, indeed, lots of different theories as to the development of the book.
I cannot find, however, anywhere that gives a satisfactory argument for the unification of the book of Daniel. I presume there must be one and so I ask here: what is the evidence, internal and/or external, that Daniel was comprised by a single author?