What is mass?

Mass, on OPH's reading, is a standing cost, the upkeep a pattern demands to stay in one piece while its records go nowhere. In 1834 John Scott Russell chased a heap of water along a Scottish canal. The water kept changing and the shape traveled. A particle is a pattern of that kind, a knot in how records pass from patch to patch, and repair, the step where neighbors settle a disagreement, works on it at every event without flattening it. The proposal is that a balance reads that bill divided by the square of the speed of light. In a proton the three quarks account for about one percent of the mass, and the rest is upkeep. Matching this upkeep to measured rest mass and to gravity's source is still open. If one entry sets both how hard a thing is to push and how strongly it bends the geometry, those two masses agree, and equivalence tests at one part in 10¹⁵ are the bar it has to clear.

Also in Chapter 18: Why Does Anything Have Energy?, Chapter 32: Why Does Anything Move?, Chapter 28: What Is a Particle, What Is a Wave, and Why Does It Look Like Both?.