重力場と物質量

I am uncertain as to how Einstein can justify the statement that the gravitational mass of a body is equivalent to its inertial mass. 重力場を磁場に比べます。　　Just as a magnetic field causes action to take place on a piece of iron indirectly, so the gravitational field around a given mass causes action to be applied to a given object. Empirical evidence shows that bodies accelerated by a gravitational field have acceleration which does not depend on the material or physical state of the body.
He then cites two equations for gravitational force:
F = inertial mass*acceleration
F = gravitational mass * intensity of field
Equating these two, acceleration = (gravitational mass/inertial mass)*intensity of field
For a given field, the acceleration is the same for any material, thus, the ratio of gravitational to inertial mass must be constant. Einstein claims that suitable units can be chosen to make such a ratio be unity.