In section 21.2, Keynes compares the factors determining price in a given industry, and those determining price across industry as a whole, apply the same logic to both, i.e. that prices are determined by the cost of the factors of production, and the scale of output. He says that both the costs of the factors of production and the scale of the output (i.e. the volume of employment) depend on demand (here I take him to mean effective demand). He also says we need new language and ideas to deal with aggregate demand across industry as a whole.