Classical economists deny the possibility of long-term underspending, which is a level of spending insufficient to purchase a full-employment output, due to Say's Law. Developed by the nineteenth-century French economist J.B. Say, Say's Law states that the very act of the producing goods generates an amount of income equal to the value of the goods produced (or in other words, the very act of producing goods requires spending that will later be used to buy those goods). In other words, the production of any output automatically provides the income needed to take that output off the market-Supply creates its own demand.
To illustrate Say's law, let's use a barter economy as an example. Barter is the exchange of one good or service for another good or service. For example, a shoemaker produces and supplies shoes as a means of buying or demanding other goods, such as shirts, which are produced or supplied by other workers. The shoemaker's supply of shoes is his demand for other goods. The same works for the entire economy. Hence, demand must be the same as supply.