title: The sources of Schwinger's Green's functions.
abstract: Julian Schwinger's development of his Green's functions methods in quantum field theory is placed in historical context. The relation of Schwinger's quantum action principle to Richard Feynman's path-integral formulation of quantum mechanics is reviewed. The nonperturbative character of Schwinger's approach is stressed as well as the ease with which it can be extended to finite temperature situations.
authors.affiliation: Dibner Institute, Massachusetts Institute of Technology, Cambridge, MA 02139, USA. schweber@fas.harvard.edu
authors.forename: Silvan S
authors.initials: SS
authors.lastname: Schweber