Saint Kitts and Nevis Map for The equation simply shows that what is spent must necessarily equal what is sold. This equation is due to Simon Newcomb and Irving FISHER in another variation. (The original form considered the transactions of quantities of goods traded for money rather than the output.) The equation, however, can be used to form a relation among the variables if it is considered as function. The causal relation among the variables is from left to right. The right hand side, PY, is nominal income and it is determined by the quantity of money and its velocity. (M and P are considered at points in time, while V and Y are specified over periods of time or per unit of time. Saint Kitts and Nevis Map 2016.