A Student’s Guide to Einstein’s Major Papers ， Sec. 5.1.2
In 1907, he published a review of the special theory of relativity, entitled “On the Relativity Principle and the Conclusions Drawn from It,” in Jahrbuch der Radioaktivit ̈at und Elektronik.7 In the last section of the paper, entitled “Principle of Relativity and Gravitation,” he writes:
［W]e shall therefore assume the complete physical equivalence of a gravitational field and a corresponding acceleration of the reference system.
The equivalence of a gravitational field and an accelerating reference frame became known as the strong equivalence principle. This one assumption (strong equivalence principle) started Einstein on his way to answering the aforementioned “lingering questions.” But it took ten years, from 1905 to 1915, for Einstein to arrive at the completed form of the general theory of relativity.