The Modem Theory of pricing of factors of production also known as "Demand and Supply Theory" gives a satisfactory answer to the problem of determining factor prices. According to the theory just as the price of a commodity which is determined by the forces of demand & supply, similarly the price of a factor of production is also determined by the demand for that factor and its supply.
Even before the publication of Johan Maynard Keynes book, some economists were of the view that income and consumption were functionally related. However, it was Keynes who first stressed the importance of the positive functional relationship between aggregate income and aggregate consumption. The consumption function c = f(y)