Marshall: According to Marshall the interest rate is the price paid for the use of capital. This rate of interest is determined by the equilibrium formed by the interaction of the aggregate demand for capital; and its forthcoming supply.
Taussig: According to Taussig, the interest rate is determined at the level where the marginal productivity of capital equals the marginal instalment of saving.
John Maynard Keynes: According to Keynes the interest rate definitely influences the marginal propensity to save. This savings is also linked to the level of income. Hence it is concluded by Keynes that the rate of interest should be at a point where the demand curve for capital at different interest rates intersects the savings curve at a fixed income level.
The Classical theory of interest defines the rate of interest as the element that equates savings and investment. Here investment is nothing but the demand for investible resources and savings is their supply. The rate of interest that is determined by the interaction of investment and savings is the price of the investible resources. Proponents of the classical theory of interest have different ways of looking at the theory and they may be explained as under:
Criticism of the Classical Theory of Interest
The fact that the demand for capital , the effect of interest on savings out of a fixed income level are all given, there should be a strong correlation between the interest rate and the income level.
If the interest rate, the demand for capital and the sensitivity of the marginal propensity to save to a change in the interest rate are all given then the income level would be the factor that would equate savings with investment.
Again according to the classical theory, if the demand curve for capital shifts or both shift, then the new equilibrium rate of interest would be determined at the new point of intersection. This concept is criticised to be totally wrong since the constancy of income does not tally with the notion that the two curves shift independent of each other. The shift of either of the two curves would change the income level and hence the entire assumption of fixed income level breaks down.