It's clean-up time again.

The experiment in part 5 could lead to some questions about mystery and knowledge, so let's clear those up right away.

1. If it's all in your head, doesn't that mean you can live in any reality you want, that reality is just whatever we imagine it to be?
   
   No, it doesn't. The world you create is all in your head, in the sense that you construct it out of the very limited and sometimes distorted information that comes through your senses. But that inside world is still your map for navigating the outside world. In fact, that's really its only purpose. So, obviously you ought (in the "wisdom" sense of the word) to do all you can to make sure that the map matches the territory.
   
   "If you're an overnight guest, a sketch that shows the way to the restroom may be just the ticket."
   
   When I was in high school, I used to wonder, privately, whether I could walk through solid objects, if I could just get myself to "really believe" that I could. One day, I was walking across campus with a friend, deep in conversation, and failed to see a steel post, until I walked into it, and was knocked flat on my back. I realized, some time after I woke up, that I had "really believed" that I could walk through that space. The post, apparently, "really believed" something quite different.
   
   The outside world is there, and it is real. The evidence we have of it (a sudden pain in the head, stars in the eyes, a lingering headache, etc.) may all be very indirect, and may be distorted, in certain ways, but it is evidence of a reality, and we think otherwise at our own risk.
   
   The lesson we can take from the phenomenon of mystery is that we can never know the reality as it is, but only the map as we have constructed it—not that any map is just as good as any other.


2. But if all our information about the outside world is so distorted and limited, isn't it hopeless to try to know anything for sure?
   
   Yes and no. If by "know for sure" you mean know that our knowledge of the outside world is 100% accurate with absolute certainty, then it really is hopeless. Human knowledge just can't do that—outside of mathematics, where the concepts are what they are by definition. (And we can't always do it there.)
   
   But if you mean "know with a growing certainty that is good enough for our purposes," then it is not hopeless at all. We can be sure enough of many things for our purposes.
   
   If you get hungry, you can be sure enough that that big white thing in the corner of your kitchen has food in it—even though you can't be sure at all that the "whiteness" you see when you look at it is the same as the "whiteness" that I see.
   
   If you see a tiny, full-grown, person at the end of the block, and a larger, full-grown person a few feet away, you can safely interpret the size difference as distance, even though you know that the effect is only due to the way your eyes work.
   
   The fact that the real Chicago is not in any way a bunch of lines on a sheet of paper doesn't mean that a map of Chicago can't be very accurate, and the fact that the world outside is not the same as the world inside our heads doesn't mean that the inside world can't be a reasonably reliable map of the outside one.


3. So it turns out that all this stuff about "mystery" is pretty meaningless.
   
   You need to do the experiment again.


4. So a "scientific" world-view is no better than any other world-view. It's all just relative.
   
   Again, yes and no. It is all relative in the sense of what you are using the world-view for. But a scientific world-view is definitely better for scientific purposes. If you want to know how to design a plane that really flies, or what dinosaurs were like, or how we evolved on this planet, or how electrons travel through space, you're much better off viewing the world through the eyes and models of science.
   
   If, on the other hand, you want to watch a sunset without getting dizzy, or pick up your coffee mug without worrying about all the space between the electrons and protons, an everyday, run-of-the-mill world-view is superior.
   
   If you're a plumber, a diagram of the pipes in the house is the map that's most valuable; if you're an electrician, a wiring diagram; if you're an overnight guest, a sketch that shows the way to the restroom may be just the ticket.
   
   In each case, the question is what you need to do, and whether the information in the map is adequate to, and accurate enough for that purpose.


5. Then all world-views are equally good, just for different purposes?
   
   That's a tricky one. The basic answer is no. Obviously, the belief that the world is flat is not "as good" as the belief that the world is round.
   
   But of course it can depend on what the "purpose" is. Galileo advanced the idea that the earth revolved around the sun, and was imprisoned. If his purpose was to better understand the solar system, he was wise. If his purpose was to stay out of trouble, the view that the sun circled the earth might have been superior.
   
   But all world-views are not equally true (in the sense of being accurate maps).


6. If our senses are so limited, and also distort the information while giving it to us, how do we manage to know anything at all?
   
   I've already touched on the answer to that in part 3, but it's a much more interesting story than I could tell there. I'm taking it up again in part 7.