什麼是 alter ego？

那位接受鳳凰衛視訪問的副助理國務卿John Negroponte，官方網頁是這樣介紹他的職責：

副助理國務卿，擔負地一副國務卿、顧問，以及國務卿的alter ego；在國務卿缺席時擔任其署理職務；且幫助國務卿規劃和執行美國外將政策和提供對國務院的全盤督導和指揮。也不時擔任特殊之責任與督導任務。

The Deputy Secretary, John Negroponte serves as the principal deputy, adviser, and alter ego to the Secretary of State; serves as Acting Secretary of State in the Secretary's absence; and assists the Secretary in the formulation and conduct of U.S. foreign policy and in giving general supervision and direction to all elements of the Department. Specific duties and supervisory responsibilities have varied over time.。

問題是：何謂alter ego？

ego是自我、自我中心，alter是改變、矯正。alter ego是提供諍言，避免外交部長的決策無意間落入偏見或個人情緒之中，從而損害國家利益嗎？

若是這樣，等於是一種以制度來排除個人因素干擾的決策設計。