I cannot get the essence of Mach's Principle.
In my undestanding, it states that the cause of the so-called "fictitious forces" which arise in non-uniform motion has to be found in the relative motion between the object under analysis and all other masses in the Universe.
Now, we now that the fictitious forces depend on the non-uniform motion of the OBSERVER and on the inertial-mass of the object under analysis --> the object under analysis contributes to these forces only with its mass, not with its relative motion.
So, I can't understand the conclusion: "The inertia of any object depends in its relative motion with respect to all other masses".
It seems to me that a more appropriate version should be: "The inertia of an object depends on the relative motion of the OBSERVER with respect to all other masses".
But I am sure I am missing something.
Can you please help me understand?