A review of theories of ferroelectricity and of experimental data on ferroelectric ceramics leads to the conclusion that an analytic theory of such ceramics on microscopic basis does not seem feasible. A thermodynamic continuum theory is developed here, which is based on the postulated isomorphism, relevant to small signals only, between a polarized ceramic and one under bias fields near the limit of its nonpolar state; this bears analogy to Mueller's classical theory of Rochelle salt. The results of the theory include an elastic relation, which supplements existing experimental data, and a piezoelectric one, which relates to retained polarization and which agrees with measurements.