<今日の進捗>
Book8 - 67 (Have you ever sat at your desk at school and just been blindly staring at the blackboard trying to understand what the teacher was saying?)

<オリジナル回答>
Is it polite to peek at other people? Why not?
No, it isn’t polite to peek at other people because it is believed to be bad manners.

【解説】
it is believed that は~だ信じられているという意味です。it is said thatと同じ構文です。