In contrast, the economic effects of the upload scenario (or the infinite immigration in the thought experiment) are much clearer, since these feature a few simple effects that are strong enough to dominate everything else.

The only effect that I discern that dominates everything else involves hitting the Malthusian limit - which I have already allowed is a strong argument (though not, I think, decisive - but I'm putting off that discussion for some other time). The other elements of the argument look to me like a question of ignoring the unseen, of assuming that the non-obvious is trivial.

In the first case, a straightforward analysis leads to the classic Malthusian scenario, i.e. both human and robot wages are reduced to the barest subsistence

Again my response to mention of the Malthusian scenario is that I want to put off discussion of that. However, the first case as I intended it (without cheap duplication) was essentially what we have now, which is not a Malthusian scenario. I assumed all costs are the same as for humans, including duplication. So, we would simply have two kinds of human, a flesh one and a silicon/metal one (say).

Cheap duplication is the key factor, not low resource use or high productivity, because duplication is, of course, the mechanism by which a population reaches the Malthusian limit. Low resource use and high productivity don't have any clear effect one way or another, because consider the following two scenarios:

1) You have a trillion minds in a cube (hence: low resource use and high productivity).

2) You have a cube-shaped portal to another world, and on that other world there are a trillion minds.

The scenarios are (from your point of view) effectively identical. But the second scenario is just the international trade scenario. Free trade is usually better, draft horses notwithstanding.

Suppose that I were living in Manhattan and there were no Japan in the world. Then one day, I find a box, and inside that box is Japan (in fact the box is a portal to Japan which is on some other world). So now the population of Manhattan is half the (previous) population of the US, because of the people in the box. The net economic impact is positive, for the same reason that the impact of trade with Japan is positive. Rent goes up in Manhattan a bit, because of the advantage of being near the portal to Japan. But the higher rent necessarily is counterbalanced by the higher advantage of being near the portal since that is the reason for the higher rent, so that the net effect is not obviously either positive or negative (I could argue that it is actually positive). Notice that at this point there is not necessarily any displacement of Americans out of the economy, even though there are 150 million minds in a box. Displacement doesn't even begin at this point, even though the population of the box is comparable to the population of the US (i.e. half).

If this is about right, then everything happens at or near the Malthusian limit. That's what we need to look out for. Not merely the existence of masses of uploads, so long as the Malthusian limit remains far.

The cost of capital becomes equal to the cost of land (which in economic parlance also includes other natural resources) necessary to produce it

Let's specify the scenario more explicitly. We assume the ghosts are completely friendly, innumerable, will do anything we want for free, but need powered bodies to do it (they have only the minutest ability to direct physical events). I think this comes closest to the upload scenario. (If we assume the ghosts have significant psychokinetic power then the scenario is I think very different from the upload scenario). The ghosts are essentially Indian subcontractors, only much cheaper (free) and much more numerous (infinite).

Immediately I think we can see that there are fairly severe bottlenecks on the ability of uploads, sorry, ghosts to direct significant physical activity. There may be infinitely many ghosts, but there are at any given time only so many powered bodies for them to direct. Alongside these powered bodies directed by external ghosts, there are powered bodies directed by internal ghosts - namely, human bodies, which have their own ghosts. There is no upper limit on the mental work that ghosts can do, but there is a severe limit on the physical work that ghosts can do no matter how many ghosts there are. So we would have an economy which was essentially all a mind-work economy, with only a minuscule fraction (zero percent, considering the infinity of minds) of the population (human or ghost) doing any physical work.

Anyway, for there to be any Malthusian result, it seems to me that it would have to involve competition for resources between human bodies and robot bodies, not between humans and ghosts directly. But I wanted to discuss events prior to hitting any Malthusian limit.

So, all a person has to do to get a ghost to help him is to build a robot with a ghost interface and supply the robot with energy. One more specification - we suppose that ghosts will help whoever owns the bodies (that easily takes care of the decision about who they help).

In principle, once a person owns a number of ghost-directed robot bodies, the bodies can do all the work required to keep themselves (and him) alive and might furthermore be able to increase their own number (by buying raw materials on the market and constructing another body, which can then be inhabited by a new ghost).

For a long time, until the Malthusian limit is reached, it's not obvious that this would significantly affect the employability of humans. Some humans would create robots and get their robots to work for them, but not all humans would have robots, and those humans would have to trade with each other as usual. And even the humans with robots would have to trade with the wider economy to get raw materials (just as slave plantation owners did), and therefore probably trade with robotless humans. After a long time, a very long time, a Malthusian limit might be reached, but the unemployability of people before that happens seems to me to be greatly exaggerated.

However, Manhattan is situated right next to a vast and much less densely populated continent from which it's cheap to bring stuff, so that food prices in Manhattan reflect the farming land rent in these neighboring places, not Manhattan itself. If the land rents in the whole world were as high as in Manhattan, you bet there would be mass starvation.

You are again assuming that the Malthusian limit is already reached. You have relied over and over on the Malthusian argument, which in my original comment - the one that you objected to - I had already acknowledged as strong (and as not specific to uploads), and I had already said that I was not critiquing it (yet).

Initially, long before the Malthusian limit is reached, it makes sense to situate the uploads in a highly populated area, like Manhattan (a cousin of mine explained that companies are buying warehouses near Wall Street and filling them with computers, because light speed is a limiting factor; it's no good to have the trading computers situated far from Wall Street). And the effect of placing the uploads in Manhattan should be much like the effect of turning a city into an international port - which raises the local rents high only to the extent that it is made more worthwhile to be close to the port, so that the net effect of the raised rents is not obviously negative (in fact I would argue positive). Far from Manhattan rents would not be much affected, and meanwhile people would benefit to some degree, just as they would increased trade from a port.

It would be a long time before the whole world turned into one large city.

Adding one new upload box is a bit like adding one new port. Imagine that every day somebody opens a new port to a new Japan on a new planet. What's the effect? Well, suppose that there is already five ports open to five Japans within a ten mile radius, and somebody opens a new port to a new Japan right next door. You ask me, this has the aroma of diminishing marginal returns about it. The port owner tries to profit from trade to a Japan via his port, but the nearness of the other ports (and the existence of hundreds or thousands of ports further away) means that he can't charge monopolistic prices. The amount of profit that a person can make from his port to a new Japan rapidly approaches the cost of setting up the port, possibly long before the countryside is completely covered with ports to Japans, and beyond that point there is no net profit to building yet another port to yet another Japan. Since that happens long before ports to Japans completely cover the landscape, then there is still much land left over for people to live on.