  We perform a short comparison between the local and linear constitutive
tensor $\chi^{\lambda\nu\sigma\kappa}$ in four-dimensional electrodynamics
(Sec.2), the elasticity tensor $c^{ijkl}$ in three-dimensional elasticity
theory (Sec.3), and the DeWitt metric $G^{abcd}$ in general relativity, with
${a,b,\dots=1,2,3}$ (Sec.4). We find that the DeWitt metric has only six
independent components.
