Say’s Law starts from the proposition that demand is constituted by supply. This is excerpted from Francisco’s money speech from Atlas Shrugged.
“So you think that money is the root of all evil?” said Francisco d’Aconia. “Have you ever asked what is the root of money? Money is a tool of exchange, which can’t exist unless there are goods produced and men able to produce them. Money is the material shape of the principle that men who wish to deal with one another must deal by trade and give value for value. Money is not the tool of the moochers, who claim your product by tears, or of the looters, who take it from you by force. Money is made possible only by the men who produce. Is this what you consider evil?
“When you accept money in payment for your effort, you do so only on the conviction that you will exchange it for the product of the effort of others. It is not the moochers or the looters who give value to money. Not an ocean of tears nor all the guns in the world can transform those pieces of paper in your wallet into the bread you will need to survive tomorrow. Those pieces of paper, which should have been gold, are a token of honor – your claim upon the energy of the men who produce. Your wallet is your statement of hope that somewhere in the world around you there are men who will not default on that moral principle which is the root of money. . . .
Money is the barometer of a society’s virtue. When you see that trading is done, not by consent, but by compulsion – when you see that in order to produce, you need to obtain permission from men who produce nothing – when you see that money is flowing to those who deal, not in goods, but in favors – when you see that men get richer by graft and by pull than by work, and your laws don’t protect you against them, but protect them against you – when you see corruption being rewarded and honesty becoming a self-sacrifice – you may know that your society is doomed. . . .
“If you ask me to name the proudest distinction of Americans, I would choose – because it contains all the others – the fact that they were the people who created the phrase ‘to make money’. No other language or nation had ever used these words before; men had always thought of wealth as a static quantity – to be seized, begged, inherited, shared, looted, or obtained as a favor. Americans were the first to understand that wealth has to be created.
I don’t think productive effort is the whole of morality which means that much of what I believe on the moral plane differs from what she believed. But at the centre of an exchange economy is the role of money whose value can only come from the production of the goods and services the money earned can be used to buy. If the money you receive is not for the production of saleable output, then the money you spend will limit what those who actually have been productive can buy.