2 4 2 D O C U M E N T 1 3 8 D E C E M B E R 1 9 2 5 The criterion that a successful unified field theory has to allow for solutions capable of represent- ing particles was first established in Einstein and Grommer 1923a, 1923b (Vol. 13, Doc. 12). In Ein- stein 1924d (Vol. 14, Doc. 170), Einstein presented a programmatic statement about the scope of a unified field theory, namely, to derive from the field equations the existence of (positive and negative) electrons as singularity-free solutions, their equations of motion, and their quantum behavior. For a brief description of this program, see also Einstein to Michele Besso, 5 January 1924 (Vol. 14, Doc. 190). A reference to the new quantum mechanics of Werner Heisenberg, Max Born, and Pascual Jor- dan, as presented in Heisenberg 1925, Born and Jordan 1925b, and Born, Heisenberg, and Jordan 1926. The latter paper’s proofs were sent to Einstein (see Doc. 112). Marco and Tycho Besso. Eduard Einstein. Anna Besso-Winteler and Vero Besso.