Understanding 自分は in this sentence (and other potential gross misunderstandings)

Question

受話器を置き、口ひげをなでながら、ダーズリー氏は考えた。
He put the receiver down and, while stroking his moustache, Mr Dursely thought.
———まさか、自分はなんて愚かなんだ。
I'm being stupid

I'm really stuck on the line in bold.

So I guess I can translate まさか as "really". I think it intensifies the rest of the sentence. Is this right?

I think なんて in this case is "how..." so まさか、なんて愚かなんだ would be "Really! How stupid I am!"

So, if what I've got so far isn't gibberish, I don't know how to fit in the 自分は part. Literally I suppose it would be "Really! As for myself, how stupid I am!".

But, is 自分は really necessary? Why? How should I think about it?

Answer 1

Translating this まさか as "Really!" is not wrong, but may be a bit too weak. I feel it's closer to "That can't be!", "Unbelievable!", "I never dreamed of it!", etc. It's not an intensifier but an interjection-like expression on its own. That is, it's not directly connected to the remaining part of the sentence.

自分は is necessary in this case. This is because まさか works as a comment about the surprising news he just heard. Without 自分は that explicitly switches the topic, the last half of the sentence would mean "How stupid it/he/she is!", referring to the news. Compare:

まさか、なんて愚かなんだ!
Unbelievable! How stupid! (←this refers to the news itself)

まさか、自分はなんて愚かなんだ!
Unbelievable! How stupid I am! (←he is disappointed at himself because he could not think of the possibility of what he heard)