The crucial thing to understand here is that these likely short-run benefits from the new phone have almost nothing to do with how good it is — with how much it improves the quality of buyers’ lives or their productivity. Such effects will kick in only over the longer run. Instead, the reason JPMorgan believes that the iPhone 5 will boost the economy right away is simply that it will induce people to spend more.

And to believe that more spending will provide an economic boost, you have to believe — as you should — that demand, not supply, is what’s holding the economy back. We don’t have high unemployment because Americans don’t want to work, and we don’t have high unemployment because workers lack the right skills. Instead, willing and able workers can’t find jobs because employers can’t sell enough to justify hiring them. And the solution is to find some way to increase overall spending so that the nation can get back to work…

But why suffer through years of depressed output and high unemployment while waiting for enough obsolescence to accumulate? Why not have the government step in and spend more, say on education and infrastructure, to help the economy through its rough patch? Don’t say that the government can’t add to total spending, or that government spending can’t create jobs. If you believe that the iPhone 5 can give the economy a lift, you’ve already conceded both that the total amount of spending in the economy isn’t a fixed number and that more spending is what we need. And there’s no reason this spending has to be private.