The classical theory of interest, a cornerstone of classical economics, explains the determination of the interest rate based on the interaction of savings and investment.
According to this theory, the rate of interest is not determined by the quantity of money (as suggested by the quantity theory of money) or by psychological factors alone. Instead, it is determined purely by the interplay of the supply of savings and the demand for investment.
The classical postulate states that:
The equilibrium interest rate is established at the point where the quantity of savings equals the quantity of investment demanded. Therefore, both savings and investment are fundamentally determined by the rate of interest.