Eric Schwitzgebel is reading Stannis Lem’s novel ‘Solaris’ and discovers in the novel a test of the existence of the external world:
I instructed the satellite to give me the figure of the galactic meridians it was traversing at 22-second intervals while orbiting Solaris, and I specified an answer to five decimal points. Then I sat and waited for the reply. Ten minutes later, it arrived. I tore off the strip of freshly printed paper and hid it in a drawer, taking care not to look at it…. Then I sat down to work out for myself the answer to the question I had posed. For an hour or more, I integrated the equations….
If the figures obtained from the satellite were simply the product of my deranged mind, they could not possibly coincide with [my hand calculations]. My brain might be unhinged, but it could not conceivably complete with the Station’s giant computer and secretly perform calculations requiring several months’ work. Therefore if the figures corresponded, it would follow that the Station’s computer really existed, that I had really used it, and that I was not delirious (1961/1970, p. 50-51).
This is similar to a test Schwitzgebel describes on his blog.
While these tests are interesting, I don’t see how they conclusively prove that solipsism, the belief that we alone exist and all else is an illusion, is false. It seems like for any test that we could construct, our very evaluation of that test would be based on sensory inputs, which might themselves be an illusion. But maybe I’m missing something?
Ultimately, I think we all have to take it on faith that the external world exists. It certainly doesn’t pay to ignore what looks like the external world, since doing so can cause substantial grief and pain. Maybe that grief and pain is also an illusion, but I find illusory pain just as…painful as the real thing.