sine qua non       
noun [ S ]  formal uk/ˌsɪn.eɪ kwɑː ˈnəʊn/ us/ˌsɪn.eɪ kwɑː ˈnoʊn/
 必要条件,必备条件
a necessary condition without which something is not possible
  An interest in children is a sine qua non of teaching. 喜欢孩子是从事教育工作的必要条件。 
 
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