All of us can recognize an arch composed of rectangular blocks.
But also, we all can imagine how it would look if its top were replaced by a three-sided block.
How could a program or mind ‘imagine’ things that are not present in the scene? We could do this by ‘envisioning’ a change at any perceptual stage!
Making changes at very low levels: In principle, we could make a new image by changing each spot of the retinal picture—but in practice, such changes would need huge computations. Also, if you wanted to shift your point of view, you’d have to compute the whole image again. Worse, before you could do such a computation, some part of your mind would first have to know precisely what that picture describes. But to do this you’d already have to represent this at some higher descriptive level—but then, why do all that calculation?
Making changes at intermediate stages: One could change, not the picture itself, but parts of higher-level descriptions. For example, at the level of Region-Finders one could change the name of that top block’s front face from “rectangle” to “triangle.” However, this would cause trouble at other levels, because that triangle’s edges would not have the proper relations to the edges of regions that neighbor on it.
Below we’ll see that it would be better to replace the whole block at the higher Object-Finder level.
Visualizer: I sometimes have troubles like that in my mind. When I try to imagine a triangular shape, I know where its three lines should appear, but I ‘see’ them as nebulous, glowing, streaks whose ill-defined ends many not properly meet. When I try to correct this by ‘pushing’ a line, it abruptly moves with some constant speed that I cannot change—and when I tell that line to stop, it tends to keep moving anyway (though, strangely, it never gets far away).
That person was trying to change descriptions but had trouble maintaining the proper relationships between their parts. Imagining is somewhat like seeing, except that when we alter internal representations, they may not maintain their consistency. A real object can’t move with two different speeds at once, nor can two real lines both intersect and not meet—but imagination has no such constraints. Of the clear affront
Making changes at the higher semantic levels: You could imagine replacing the top of that arch by merely changing the name of its shape, e.g., by replacing rectangular by triangular in, “A rectangular block supported by two upright blocks.”
Now think about how efficient this is! To make such a change at the sensory level, you would have to alter thousands of ‘pixels’—the items of data that make up a picture—whereas you need only to change a single word when you work at an abstract linguistic level, to represent an entire thing by one or only a few compact symbols. Of course, those symbols are useless unless each one is connected to structures that give more details or ‘meanings’ to them.
Our Builder system could do such things by making changes in what we call “Semantic Networks.” For example, it could represent a three-block Arch by describing relations between three blocks.[88] Then, to ‘imagine’ a triangular block on the top, Builder needs only to change a single link:
To make such changes at earlier stages would involve too many details. If you only recorded a picture-like image, then it would be hard to change any part. But at the higher ‘semantic’ levels, you can more easily make more meaningful ways changes. For example, when you describe “a lying-down block supported by two upright blocks,” you need not specify the viewer’s perspective, or even say which parts of the scene are in view. Consequently that same description applies equally well to all these views:
If we substitute ‘object’ for the word ‘block,’ then our network would describe yet more situations, including these:
This shows how convenient are ‘abstract’ descriptions. Sometimes the word ‘abstract’ is used to mean ‘intellectually difficult’—but here it has almost the opposite sense: abstract descriptions are simpler when they suppress details that are not relevant. Of course, descriptions must not be too abstract: as when you ask someone for advice, and they give you a useless reply like, “If you want something, do what will get it for you.”
We’ve discussed how we might imagine visual scenes by constructing “simuli” inside our minds. We do similar things in other realms. Perhaps some chefs imagine new textures and tastes by changing their lower-level sensory states—and perhaps some composers imagine the sounds of new kinds of instrumentations—but such thinkers might also achieve such effects by making smaller changes at higher levels of representation, and thus evoke delight or disgust without constructing low-level details of those imagined musics or meals.
Drama Critic: I can clearly recollect how I felt after attending a certain performance, but I can’t remember any details at all of what that dreadful play was about.
To discuss this, we’ll coin a new word by combining “simulate” and “stimulus.” A simulus is a counterfeit perception caused by changing a mental representation. Thus in the Challenger scene of §4-7, we saw how a simulus of defeat could be used to evoke a feeling of Anger. To do this, it might suffice to represent no more than a sneer on one’s enemy’s face—with no other features of that face—for one can get by with the simplest kinds of descriptions by using the highest level abstractions.
Visualizer: When I think about my cat, its image is filled with so many details that I can visualize every hair. Would there not be a major advantage to making a real, pictorial image.[89]
Perhaps when you first imagine that cat, its surface has only a ‘furry texture’— and only when you ‘zoom in’ on it do you add more details to your mental representation. However, this could happen so quickly that you have no sense of it happening, and then it may seem to you as though you saw all those details at once. This could be an example of the illusion we mentioned in chapter §4:
The Immanence Illusion: When your questions get answered before you have asked them, it will seem that you’re already aware of those answers.
The Immanence Illusion applies not only to scenes that we imagine; we never see real scenes ‘all at once’, either, because we don’t perceive most fine details until some parts of our minds make requests for them. Indeed, recent experiments suggest that our inner descriptions of visual scenes are rarely updated in real time.[90] Chapters §6 and §8 will describe a scheme called “Panalogy” which might help to explain how our brains get such answers so rapidly.
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