The title of this thread is taken from the 1952 paper by Professor D.W.Sciama. Having just spent the lunch-hour reading it, I have a question. (I accept that I should go and do some more reading to find the answer myself, but life is short and my desire to know is strong.) To what degree of accuracy are we sure that the inertia of a body is always the same with respect to a force coming from any direction? Given that the Cosmic Microwave Background hints at areas of slightly lower mass density in the early universe, and the recent identification of large voids within which no galaxies exist might it be possible to detect a slightly different value of inertia with respect to those directions? Obviously such an effect must be extremely small otherwise all sorts of dynamic measurements would have revealed it long ago.