The Notion of Aggregate Demand Separate from Aggregate Supply Was Foreign to Pre-Keynesian Economic Thought

Moving forward a century [from when James Mill wrote], the same concept is found in the following passage from one of the most widely used economic texts ever published, in which this principle is stated in very clear terms:

It is only because our exchanges are made through money that we have any difficulty in perceiving that an increase in supply is (not 'causes') an increase in demand . . . An increase in the supply of cloth is an increase in the demand for other things; and vice versa, an increase in the supply of anything else may constitute a demand for cloth. What is divided among the members of society is the goods and services produced to satisfy its wants; and the same goods and services are both Supply and Demand. (Clay, [1916] 1924: 242)

The notion of aggregate demand separate from aggregate supply was foreign to pre-Keynesian economic thought. Aggregate demand grows at the same rate and by the same amount as aggregate supply, and will not grow unless supply has grown. It is not, however, just any production that will lead to an increase in aggregate demand. What creates demand is the production of forms of output for which enough buyers can be found whose payments cover in aggregate the entire costs of production. Only if  the goods and services produced can be sold for more than was paid for the inputs that went into their production can it be said with certainty that value has been added during the production process. Conversely, if the goods and services produced do not create more value than is used up in the production process, there can be no increase in aggregate demand because there has been no increase in aggregate supply in any relevant sense.

--Steven Kates, Free Market Economics: An Introduction for the General Reader, 3rd ed. (Cheltenham, UK: Edward Elgar Publishing, 2017), Kobo e-book.