James Clerk Maxwell
"9 coefficients determine the relationship between flux and intensity
6 of these coefficients form 3 pairs of equal quantities
3 pairs of equal coefficients will self-conjugate." [James Clerk Maxwell]

The original quote from "https://svpwiki.com/pdffiles/A_Treatise_on_Electricity_and_Magnetism.pdf - vol. I".

"The case in which the components of the flux are linear functions of those of the force is discussed in the chapter on the Equations of Conduction, Art. 296. There are in general nine coefficients which determine the relation between the force and the flux. In certain cases we have reason to believe that six of these coefficients form three pairs of equal quantities. In such cases the relation between the line of direction of the force and the normal plane of the flux is of the same kind as that between a diameter of an ellipsoid and its conjugate diametral plane. In Quaternion language, the one vector is said to be a linear and vector function of the other, and when there are three pairs of equal coefficients the function is said to be self-conjugate." [James Maxwell, A Treatise on Electricity and Magnetism - vol. I]