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Let me state it another way. If people are suddenly buying more of product X than before, then that increase in demand can be caused by only two things:
1. A decrease in demand for other goods and services, in which case demand has merely shifted.
2. An increase in the supply of other goods and services, leading to lower prices and increased purchasing power, in which case demand increased only because supply increased first.
A tax cut could easily put more money into the hands of consumers, who would then purchase more goods and services.
An increase in wages could do the same.
A redistribution of money from those who have excess to those who don't have enough could also increase realized demand. This doesn't even have to be by a government, it could be by people with excess lending to those with a shortage. Or it could be caused by my great uncle Herb passing away and me inheriting all the money that he had been hording.
Or even foreign demand for US products could increase (maybe a weaker dollar), and that would cause an increase in business sales.
There are probably dozens of other scenarios which could cause an increase in realized demand that I'm just not smart enough to think of, but any of the above would cause an increase in production.
I think the confusion lies in conflating dollars with actual wealth, and that someone the existence of more dollars means there will be more wealth. This is simply not true. But yes, it appears we will once again agree to disagree, as we do every time this discussion arises.
No, I absolutely understand that money isn't wealth.
You are thinking that if people spend more money, inflation will be created as a result, which would wipe out 100% of the additional purchasing power. That typically doesn't happen, although it can happen if there is some sort of restriction in the production of goods and services. Your agriculture example would probably one of those situations. But if my business increased in 2015 over 2014, unless all my competitors increase their prices, and unless there is some sort of cost-push inflation, I would have no reason to increase prices, because my business, like virtually every other business, has the capacity to produce more, without increasing capital investment. I can simply hire more workers, or work longer hours myself, or work my few employees longer hours.
I think where a lot of people get confused is two things:
1) people assume that every business operates at full capacity all of the time. That's simply not true. Any well run business operates below capacity most of the time, so that it can meet demand when it has a temporary increase in demand. I've held a dozen or so jobs in my life, I've never worked at a place that couldn't produce more.
2) economics is boring for most people, and so is math. In our high school economics class, the football coach tells reads out of the textbook that "inflation is caused by too much money chasing too few goods". But we tend to zone out after hearing the first few words, so all we really comprehend is "inflation is caused by too much money". And since we are already assuming that no more goods can be produced (at least without a significant capital investment and time), we ignore the fact that producers strive to meet demand. There is really never "too much money". I've never had "too much money", have you? The only time that more demand results in higher prices is when production can't meet demand, which really isn't that common. When was the last time you went into a Walmart and their shelves were bare? When was the last time that the car dealer sold out of cars and couldn't get anymore? When was the last time that China refused to sell us trinkets? Particularly when unemployment is above the full employment level, an increase in demand does not cause inflation.