Why is "themselves" necessary in "they themselves cannot be"? If I say just "they cannot be", does the meaning get changed?
Great question. Removing "they" from the sentence would not change the meaning. However, the use of "they themselves" is there for emphasis. "-self" pronouns are actually called emphatic pronouns and they're used to emphasize the subject of the sentence. Ex: They themselves didn't know the answer to the question. Ex: They didn't know the answer to the question.