Cosmology

Acceleration seems . . . to be measured relative to the distant galaxies - that is, relative to the average distribution of all matter in the Universe. It is as if, when you try to push something around, it takes stock of its situation relative to all the matter in the Universe, and responds accordingly. It is somehow held in place by gravity, which is why gravitational and inertial mass are the same. . . . Inertia is . . . produced by the response of a material object to the Universe at large.
		p. 227-228
John Gribbin	Schrodinger's Kittens and the Search for Reality Back Bay Books, 1995
Gribbin notes that this way of looking at inertia is called Mach's Principle. It is named after Austrian physicist Ernst Mach, "whose name is immortalized in the number used to measure speeds relative to the speed of sound" (p. 228).