The question of the origin of the inertia of matter presents a very curious \index{Inertia!electromagnetic origin of}% paradox. We have to distinguish--- \begin{align*} &\text{the invariant mass~$m$ arising from the \PadTxt[l]{coordinate}{invariant} density~$T$, and} \\ \index{Invariant mass}% &\text{the \PadTxt[l]{invariant}{relative} mass~$M$ arising from the coordinate density~$T^{44}$.} \end{align*} As we have seen, the former cannot be attributed to the electromagnetic field. But it is generally believed that the latter---which is the ordinary mass as understood in physics---arises solely from the electromagnetic fields of the electrons, the inertia of matter being simply the energy of the electromagnetic fields contained in it. It is probable that this view, which arose in consequence of J.~J. Thomson's researches\footnotemark,\footnotetext {\Title{Phil.\ Mag.}\ vol.~11 (1881), p.~229.} is correct; so that ordinary or relative mass \PageSep{184} may be regarded as entirely electromagnetic, whilst invariant mass is entirely non-electromagnetic. How then does it happen that for an electron at rest, invariant mass and relative mass are equal, and indeed synonymous? Probably the distinction of Maxwellian and non-Maxwellian stresses as \index{Non-Maxwellian stresses}% \index{Stress-system!non-Maxwellian}% tensors of different natures is artificial---like the distinction of gravitational and inertial fields---and the real remedy is to remodel the electromagnetic equations so as to comprehend both in an indissoluble connection. But so long as we are ignorant of the laws obeyed by the non-Maxwellian stresses, it is scarcely possible to avoid making the separation. From the present point of view we have to explain the paradox as follows--- Taking an electron at rest, the relative mass is determined solely by the component~$E^{44}$; but the stress-components of~$E^{\mu\nu}$ make a contribution to~$E$ which exactly cancels that of~$E^{44}$, so that $E = 0$. These stresses are balanced by non-Maxwellian stresses $M^{11}$,~\dots\Add{,} $M^{33}$; the balancing being not necessarily exact in each element of volume, but exact for the region round the electron taken as a whole. Thus the term which cancels~$E^{44}$ is itself cancelled, and $E^{44}$~becomes reinstated. The final result is that the integral of~$T$ is equal to the integral of~$E^{44}$ for the electron at rest.