After mulling this over the past few years, I thought of a way to frame how Say's Law appears to function: via what I'll call "equilibrium stock" and "equilibrium flow". These terms differ from the normal usage of stock and flow, which is also important since in traditional definition, stock lacks a recognizable alignment with flow at a macro level.
In GDP measure, as far as I can tell (please someone correct me if I'm wrong) we don't yet have a way to conceptualize how differences between stock and flow affect output. Why? Because traditional stock accumulation has little definitive active economic context. Plus I'm still crazy enough to imagine that a better macroeconomics can keep our society from falling off the edge of a cliff, given events of late. Here's Wikipedia:
A stock variable is measured at one specific time, and represents a quantity existing at that time (say, December 31, 2004) which may have accumulated in the past. A flow variable is measured over an interval of time. Therefore, a flow would be measured per unit of time (say a year). Flow is roughly analogous to rate or speed in this sense.
For example, U.S. nominal gross domestic product refers to a total number of dollars spent over a time period, such as a year. Therefore it is a flow variable, and has units of dollars/year. In contrast, the U.S. nominal capital stock is the total value, in dollars, of equipment, buildings, inventories, and other real assets in the U.S. economy, and has units of dollars.Whereas, equilibrium stock would consist of product which is derived via internalized costs, for product which has not been previously sold (the time arbitrage I've suggested would function this way as well). Theses costs can be readily discerned, and while they generally take place within one institution, in some instances they may be coordinated with other institutions, so long as product costs are met as product enters the marketplace.
Product which takes place in these circumstance, establishes the perimeters of a given marketplace equilibrium, whereby additional flows take place. In other words, output gains for this "first marketplace" position, is when increased equilibrium supply can meet (the overgeneralized Say's Law interpretation of) increased equilibrium demand.
Unlike equilibrium stock, equilibrium flow is externalized, so that cost and output patterns cannot be readily discerned at the time of marketplace entry. While I've emphasized the example of government compensation for time based product, even government expense for tradable sector product is subject to this equilibrium constraint, in terms of already existing aggregate spending capacity. Distinctions such as these could apply to endless debates re government restrictions on growth potential, since plenty of private activity is also further riffs on equilibrium flow.
For purposes of GDP, equilibrium stock would categorize what is internally or recognizably purchased at the time of marketplace entry in the previous year, because this holds important clues about existing flows, which are important for monetary representation as the currently existing agreements which economic participants seek to uphold. Why return Say's Law to ongoing dialogue? It's not enough to explain that new income generates new income (arguments which essentially "replaced" Say's Law), without considering whether income derives as a source of equilibrium stock or flow, in the previous year.
One reason why a "natural" interest rate can appear as though negative: far more economic activity may take place in terms of equilibrium flow, instead of as equilibrium stock. Nevertheless, there are important reasons why some prefer less output, if that is necessary to control what derives from a dependent, or secondary, market position. Importantly, supply side factors which control how knowledge is utilized in the marketplace, carry more ongoing responsibility for employment limitations, than the missteps of fiscal and monetary policy.
Consider that the only equilibrium stock component of real estate, is new building construction. So far as real estate is concerned, mortgages, "reused" land, and rent are all components of equilibrium flow, since they function as claims on already existing income and/or resources. Yet even the equilibrium flow element of land costs, simply tells a value story about scarcities in productive agglomeration. Indeed, this is why I have doubted the efficacy of land taxation as a reliable revenue source.
Also note that incentives for the entrepreneurs of non tradable sector activity in a dependent market position, are not the same as incentives for tradable sector entrepreneurs, who gain from output expansion. In part since a dependent market position only encourages non tradable sectors to limit both supply and employment, those earlier supply and demand structures appeared less relevant, as non tradable sector activity began to dominate the marketplace.
Fortunately, a marketplace for time value, or time arbitrage, could restore entrepreneurial incentive for both output and employment. I believe that time arbitrage could give Say's Law greater validity in the future, than what has been possible since the secondary market dominance of the 20th century.