A microscopic mechanism of ferroelectric instability is proposed, based on the idea that in the ferroelectric phase, the main state of the system should be a state with a nonzero constant electric field inside the crystal. It is shown that this approach can provide a unified description of the physical mechanism for transitions that have traditionally been considered transitions of different types: "displacement type" and the "order-disorder" type. The dependence of free energy on polarization differs from the Ginzburg-Landau functional, although it has similar properties and is reduced to the usual Ginzburg-Landau functional near the transition temperature. Keywords: Ferroelectric phase transition.