  The question is examined, whether the formally straightforward extension of
Hooke's time-honoured stress-strain relation to the four dimensions of special
and of general relativity can make physical sense. The four-dimensional Hooke's
law is found able to account for the inertia of matter; in the flat space, slow
motion approximation the field equations for the ``displacement'' four-vector
field can encompass both linear elasticity and inertia. In this limit one just
recovers the equations of motion of the classical theory of elasticity.
