Value & Capital, CHAPTER IX, Section 7

This section provides a short summary of the three fundamental concepts used in the dynamic model, along with brief justifications and descriptions of their use.  Using the notion of a week allows the analysis "to treat a process of change as consisting of a series of temporary equilibria;  this enables us still to use equilibrium analysis in the dynamic field."  In the case of the plans that the firms (and private persons) are assumed to make each week, and that are assumed to unfold over the course of the week,

we find ourselves able to conceive of the situation at the end of the week being different from the situation at the beginning;  thus the new temporary equilibrium which is established in a second week must be different from that which was established in the first;  going on in like manner, we have a process under way.

Using the device of definite expectations enables the use of the same type of analysis as that used in the static case of determining an equilibrium for a private individual or firm.  In the dynamic case, however, we are determining the effects of both current prices and expected prices on the plans that firms and individuals make.

These three fundamental notions enable the concept of market equilibrium in the dynamic case to be explored using the "machinery" of the static case ("without abandoning our model to stationariness").