The broad notion is that “mass there influences inertia here”.
— Wikipedia on Mach principle
Mach principle states that the inertial mass of an object is due to (the gravitational influences of) other objects.
In other words, the inertial mass of an object is due to the gravitational mass of itself and the gravitational masses of other objects.
2011.09.27 Tuesday (c) All rights reserved by ACHK