For example, you know that a typical cup is about as large as your hand. So if a cup fills as much of the scene as does your outstretched hand 
, then you can reach it from where you stand. Similarly, you can judge how far you are from a typical chair, because you know its approximate size.
However, even when you don’t know an object’s size, you still have ways to estimate its distance from you. For example, if you can assume that two things are of similar size, then the one that looks smaller is further away. Of course, that assumption may be wrong, if one of those objects is a small model or toy. And also, whenever two objects overlap, then the one in front must be closer to you, regardless of its apparent size.
You can also get spatial information from how the parts of a surface are lighted or shaded, and from an object’s perspective and context. Again, such clues are sometimes misleading; the images of the two blocks below are identical, but the context suggests that they have different sizes.
If you assume that two objects lie on the same level surface, then the one that looks higher lies further away. Also, finer-grained textures look further away, and so do things that look hazier.
You can also judge a distance to an object by difference in its images in your two eyes—either by the angles between those two images or by the small ‘stereoscopic’ disparities between those slightly different images.
Also, if an object is moving, then the closer it is to you, the faster it will appear to move. You also can estimate its range by how you must change the focus of the lens of your eye.
Finally, aside from all these perceptual schemes, one frequently knows where some object is, without using any vision at all—because, if you’ve seen a thing in the recent past, its location is still in your memory!
Student: Why would we need so many different methods, when surely just two or three would suffice?
In almost every waking minute, we make hundreds of judgments of distance, and yet we scarcely ever fall down the stairs, or accidentally walk into doors. Yet each of our separate ways to estimate distance has many different ways to fail. Focusing works only on nearby things—and many persons can’t focus at all. Binocular vision works over a longer range, but quite a few people are unable to compare the images in their two eyes. Some methods fail when the ground isn’t level, and texture and haze are not often available. Knowledge only applies to objects you know, and an object might have an unusual size—yet we scarcely ever make fatal mistakes because we can use so many different techniques.
But if every method has virtues and faults, how do we know which ones to trust? The next few sections will discuss some ideas about how we manage to so quickly switch among so many different ways to think.
§8-3. Panalogy
The previous section emphasized how many different techniques we could use to accomplish the same objectives — mainly to know how far away some Object is. However, it would not help us very much to have so many methods available, unless we also had some way to switch among them almost effortlessly. This section will suggest a particular kind of machinery that, I suspect, our brains might use to do such switching almost instantly.
In Chapter 6 we mentioned that when you read the sentence, “Charles gave Joan the Book,” this can cause you to interpret ‘book’ in several different realms of thought: as an object, possession, or storehouse of knowledge.
However, having multiple representations won’t help you much unless you use the context to rapidly switch to the appropriate meaning.
Student: Why would you actually need to switch? Why can’t you maintain all those meanings at once?
There are always limits to how many things a person can do simultaneously. You can touch, hear, and see things concurrently because those processes use different parts of the brain. But few of us can draw two different things with both hands, simultaneously—presumably, because these compete for resources that can do only one of those things at a time. This section will suggest how our brains could quickly switch between different meanings.[152]
Whenever you walk into a room, you expect to see the opposite walls, but you know that you will no longer see the door through which you entered that room.
Now walk to the West wall that is now to your left, and turn yourself to face to the right; then you will be facing toward the East.
The South wall has now come into view, and the West wall now is in back of you. Yet although it now is out of sight, you have no doubt that it still exists. What keeps you from believing that the South wall just now began to exist, or that the West wall has actually vanished? This must be because you assumed all along that you are in a typical, box-like room. So, of course you knew just what to expect: all four sides of that room will still exist.
Now consider that each time you move to another place, every object you that you have seen may now project a different shape on the retinas in the back of your eyes—and yet those objects do not seem have changed? For example, although the visual shape of that North wall has changed 
, you still see it as rectangular. What makes those meanings remain the same?[153] Similarly, you now see an image of that chair in which it appears to have turned around 
—but you usually don’t even notice this, because your brain knows that it is you who has moved and not the chair. Also, you now can see the door that you entered through—yet none of this surprises you!
What if you next turn right to face the South? Then the North wall and chair will disappear, and the West wall will re-enter the scene—just as anyone would expect.
You are constantly making these kinds of predictions without any sense of how your brain keeps dealing with that flood of changing appearances: How do you know which things still exist? Which of them have actually altered their shapes? Which of them have actually moved? How do you know you’re still in the same room?
Student: Perhaps those questions do not arise because we’re seeing those objects continuously. If they suddenly changed we’d notice this.
In fact, our eyes are always darting around, so our vision is far from continuous.[154] All this evidence seems to suggest that, even before you entered that room, you have already, somehow, assumed a good deal of what you were likely to see.
“The secret is that sight is intertwined with memory. When face to face with someone you newly meet, you seem to react almost instantly—but not as much to what you see as to what that sight “reminds” you of. The moment you sense the presence of a person, a world of assumptions are aroused that are usually true about people in general. At the same time, certain superficial cues remind you of particular people you’ve already met. Unconsciously, then, you will assume that this stranger must also resemble them, not only in appearance but in other traits as well. No amount of self-discipline can keep those superficial similarities from provoking assumptions that may then affect your judgments and decisions.”
152
This switching usually happens so quickly that we don’t notice it; this is a typical instance of the Immanence Illusion [See §4-3.1.]
153
There is a detailed theory of how this works in §24.6 Direction-Nemes of
154
It was recently discovered only recently that people often do not perceive some very large changes in a scene. See [give reference] for astonishing demonstrations of this.