In section 12.1, Keynes introduces the need to better understand how the prospective yield of capital goods is ascertained. Seeing as the relation between supply price and prospective yield determines the marginal efficiency of capital (of the capital goods in question), and seeing as the relation between the marginal efficiency of capital and the interest rate determines the rate of investment, it is thus important to understand how prospective yield factors into things.
To this effect, Keynes proposes that the field of considerations informing prospective yield can be broken down into two categories: short-term expectations and long-term expectations. The former concerns the existing factors (such as existing stock and existing demand) while the latter concerns the factors likely to vary in the future (such as real wages, future demand, etc.).