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