I am curious to know the various opinions, what ideas you favor as to the cause of inertia? What could the reference frame(s) be that induces inertial effects, and how could those effects be caused? Could the reference frame be absolute space as Newton thought, or the more relativistic idea of the mass of the distant stars, as Mach thought? Any ideas as to why inertial mass is equal to gravitational mass? Do you believe that this indicates there is a relationship between the two? Lastly, would you say that Einstein incorporated Mach's ideas somewhat when he assumed that the effect of gravitation is equivalent to acceleration, because otherwise, how could acceleration produce an effect similar to gravity were it not for inertia? Lastly, if Mach was correct that the mass of the universe is the cause of inertia, why is inertia not diminished as the universe expands, since the gravitational pull of the distant galaxies would grow progressively weaker? And if there is a Mach-friendly explanation for why inertia is not diminished as the universe expands, then why wouldn't the infinite mass of an infinite universe result in infinite inertia? I am a physics layman, so please try to keep the replies to me non-technical, though I am willing to put forth the effort to struggle through more technical answers if need be. However, I will not at all begrudge you replying to each other in technical language if you want to.