This conjecture supports the existence of a field of particles in space that I’ll call the temporal-inertial (TI) field. This field provides a frame of reference for motion such that the acceleration of an object with respect to the field causes the familiar inertial reaction force. The inertial mass of a massive particle or object comprising massive particles is a measure of the object’s resistance to acceleration relative to the TI field. We can say that the inertial mass couples the acceleration of the object to its reactive force. Particles of the TI field permeate space including the space within atoms. A massive particle or object moving through the TI field at constant velocity experiences a constant flux of particles of the TI field and is unrestrained in its motion. If the object is accelerated, the object experiences a change in the flux. The inertial reaction force is proportional to the rate of change of flux. The rate of change in flux for a given acceleration is, relativistic effects notwithstanding, independent of the velocity of the particle. The basic coupling of a matter particle with the TI field is fixed and does not change with the velocity of the particle relative to the TI field. This coupling is the particle’s rest mass. As the velocity of a massive particle increases, its resistance to acceleration increases in proportion to the Lorentz factor γ. This apparent increase in mass reflects an increase in the interaction of the particle with the TI field. The relativistic effect causing the increase in resistance to acceleration is length contraction of the TI field and it produces an increase in the density of particles of the TI field encountered by the massive particle. A given acceleration of an object thus produces a change in particle flux of the TI field proportional to the product of the acceleration and the Lorentz factor γ. Relativistic mass is the measure of this total resistance to acceleration at relativistic velocity. Both mass and relativistic mass are emergent properties of matter arising from the interaction between matter particles or objects comprising matter particles and particles of the TI field.