Can someone please help me understand the second blank. And why is “utterly mysterious” the right answer and not “thoroughly commonplace”.
Thanks in advance 
I am assuming you know the block of 4 strategy as taught by greg, if not go watch it , its a short video and it is really helpful in tackling up many tc problems.
Ok now,coming to the problem, the hyphen in the sentence tells us that the first part and the second part of the sentence are related. So “rumination” in the first blank matches most with “utterly mysterious” coz the rest are completely out of the picture and “so familiar” matches with" most prosaic". This is the direct strategy in action.
But excluding the strategy if you want to make a connection, it is already indicated in the sentence that philosophy deepens your understanding, and when you ruminate on the thing you find it mysterious . Hope this answers your question