Earth is the only planet we know of where life exists. Because you've just told them! It's like when saying did you know strawberries aren't berries, but bananas are?, i didn't know if.
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This sentence is from the commentary of planet earth. In my understanding, ' as we know it ' usually follows a noun phrase and means like the building as we know it = the version/condition of. Why do you think that he doesn't know him from his schooldays means that he does know him?
Why it is know of in.
It would only have that sense if you added. Recently one of my friends told me that there is distinct difference between 'know of something' and 'know about something' expressions.
