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Between 1905 and 1907, Einstein first tried to extend the special theory ofrelativity in such a way so as to explain gravitational phenomena. This was themost natural and simplest path to be taken. These investigations did not fit inwith Galileo's law of free fall. This law, which may also be formulated as thelaw of the equality of inertial and gravitational mass, was illuminatingEinstein, and he suspected that in it must lie the key to a deeperunderstanding of inertia and gravitation. Einstein's 1907 breakthrough was toconsider Galileo's law of free fall as a powerful argument in favor ofexpanding the principle of relativity to systems moving non-uniformly relativeto each other. Einstein realized that he might be able to generalize theprinciple of relativity when guided by Galileo's law of free fall; for if onebody fell differently from all others in the gravitational field, then with thehelp of this body an observer in free fall (with all other bodies) could findout that he was falling in a gravitational field.

Einstein's Pathway to the Equivalence Principle 1905-1907