Externalism \Ex*ter"nal*ism\n. 1. The quality of being manifest to the senses; external acts or appearances; regard for externals. [1913 Webster] This externalism gave Catholicism a great advantage on all sides. --E. Eggleston. [1913 Webster] 2. (Metaph.) That philosophy or doctrine which recognizes or deals only with externals, or objects of sense perception; positivism; phenomenalism. [1913 Webster]