「西半球觀」係指將西半球的所有人民連成一氣而形成特殊關係、並使他們有別於世界其他地區的人們。美國人「西半球觀」的核心是美洲人與歐洲人「互不相屬，互不相涉」，其淵源於獨立革命時期的國族想像，與「孤立主義」相互為援，並且，其適用範圍在西元十九世紀初西班牙屬美洲殖民地爆發獨立革命風潮時被進一步擴大，以因應區域政治的變動。「西半球觀」逐漸系統化，賴有湯瑪斯‧傑佛遜之演繹，而且滲透進入門羅主義，實為美國式泛美主義的根源。如果將美國版與南美版的泛美主義回溯其最素樸的論述並加以比較，可以發現：「互不相屬，互不相涉」是兩者共通的基本原則。但是由於整合模式與範圍上有歧異，遂使兩者一開始就分道揚鑣。尤其，傑佛遜的「西半球觀」蘊含著美國優先的推想，又傾向保留美國人在美洲開拓的自然權利，就使他所主張的美洲整合，儘管仍有善意，卻也包藏著美國擴張主義與霸權發展的動機。
The Western Hemisphere idea refers to a proposition that the peoples of the Western Hemisphere are united in a special relationship to one another that sets them apart from the rest of the world. Believing that the Americans and the Europeans do not belong to and would not intervene in each other could be traced into the national imaginations during the period of the independence war and became the core of the West Hemisphere idea. Such a belief, which and isolationism supported each other, was further applied to deal the possible changes in geopolitics when Spanish American Revolution broke out in the early 19^(th) century. Then through infiltrating Monroe Doctrine the West Hemisphere idea, elaborated primarily by Thomas Jefferson, became the origin of the Pan-Americanism of the United States. Compared with the South American version by Simón Bolívar, the North American version of the West Hemisphere Idea has the same principles of ＂non-belonging and non-intervening＂. However, when Thomas Jefferson raised his West Hemisphere idea he still assumed United States' priority and tended to preserve their natural rights in other North American territories. As a result, his idea hinted their prime expansionism and hegemonism, in spite of some sympathy. The earlier distinctions between two versions of Pan-Americanism made two ways for the American integration.
