I need help understanding a passage in my textbook, where the form of hookes law in continous elastic media is explained. It says: "The absence of internal directions in isotopic matter tells us that there are only two tensors available to construct a linear relation between the the stress tensor and the strain tensor. One is the strain tensor itself and the other is the kronecker delta multiplied by the trace of the strain tensor. Consequently the most general strictly linear tensor relation between stress and strain is of the form: Here follows a linear relation between the stress- and strain tensor involving the strain tensor and its trace in separate terms. " I don't really understand all this. Why is only the strain tensor and its trace avaible for constructing a linear relation between the two tensors? Does it have to do with the fact that we would like Hookes law to be invariant under change of directions, since the symmetry dictates that there are no internal directions?