raison d'etre /ˈreɪzəʊn ˌdɛtrə, ˈrɛzɔːn/ (also raison d'être)

• 1. 

  N-SING A person's or organization's raison d'etre is the most important reason for them existing in the way that they do. 存在的首要原因 [usu with poss]

  例：

  ...a debate about the raison d'etre of the armed forces.

  ...关于武装力量存在的首要原因的争论。