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). [9]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). [10]Marco and Tycho Besso. [11]Eduard Einstein. [12]Anna Besso-Winteler and Vero Besso.