Fermi helped establish a new framework for understanding matter, based on quantum theory. This framework refines and improves traditional atomism in two crucial respects. First, the elementary constituents of matter belong to a very small number of classes, and all objects of a given class (e.g., all electrons) are rigorously identical, indeed indistinguishable. This profound identity is demonstrated empirically through the phenomena of quantum statistics, and is explained by the principles of free quantum field theory. Second, objects of one class can mutate into objects of other classes. Such mutability can be understood as manifesting interacting quantum field theory. Fermi contributed to establishing theoretical foundations for the new viewpoint, through his work on quantum statistics and quantum field theory, and to its fruitful application and empirical validation, through his work on beta decay, nuclear transmutation, and primeval strong interaction theory.