Each phase of this script, in turn, needs several steps. She could “Get to the airport” by bicycle, taxi, or bus, but she decides to drive her car, which begins with a series of subgoals like these:
When Joan reviews that airplane trip, she decides it would waste too much of her time to park the car and pass through the security line. The actual flight from home to New York takes no more than an hour or so, and the railroad trip is four hours long, but it ends near her destination—and she could spend all that time at productive work. She ‘changes her mind’ to take the train.[114]
Similarly, when Carol decides to change this 
into this 
, she will need to split this job into parts, and that will need several subgoals and scripts.
Then each of those subgoals will turn out to require several more parts and processes—and when we developed a robot to do such things, its software needed several hundred parts. For example, Add a block needed a branching network of subgoals like these:
Each of those subgoals has problems to solve. Choose must not select a block that is already supporting the tower top. See must recognize objects regardless of color, size, and shades of light—and even when they partly obscured by other objects. Grasp must adapt the robot’s hand to the perceived size and shape of the block to be moved. And Move must guide the arm and hand through paths in space that never strike the tower’s top or hit the child’s face.
How do we find out which subgoals we’ll need to achieve before we can accomplish a job? You could discover them by trial and error, or by doing experiments inside your mind, or recalling some prior experience. But perhaps our most generally useful method is to use a Difference-Engine—because every difference that this detects could thereby become a new subgoal for us.
To summarize, our idea is that “to have an active goal” amounts to running a Difference Engine-like process. I suspect that, inside each human brain, many such processes all run at once, at various levels in various realms. These range from instinctive systems that work all the time—like those that maintain our temperatures (and these are so opaque to reflection that we don’t recognize them as goals at all)— up to those highest self-conscious levels at which we keep trying try to be more like the persons we wish we were.
§6-4. A World of Differences
“Some minds are stronger and apter to mark the differences of things, others to mark their resemblances. The steady and acute mind can fix its contemplations and dwell and fasten on the subtlest distinctions: the lofty and discursive mind recognizes and puts together the finest and most general resemblances. Both kinds however easily err in excess, by catching the one at gradations, the other at shadows.”
Whenever somebody tells you a story, you react less to what each separate sentence means than to how this differs from what you expected—and this also applies to our other perceptions. For example, if you plunge your hand into a bowl of cold water, you’ll feel a severe sensation of chill—but soon this will totally disappear, just as a steady pressure on your skin will quickly seem to diminish in strength. It is the same with new odors or tastes, or with the onsets of continuous sounds: at first those sensations may seem intense but then they rapidly fade away. We have many different names for this, like accommodation, adaptation, acclimatization, habituation, or just becoming accustomed to it.
Student: This doesn’t apply to vision, though. I can look at an object as long as I like, and its image never fades; in fact, I keep seeing more features of it.
Physiologist: In fact, that image would rapidly fade if you could keep from moving your eyes, which normally make small motions that keep changing your retinal images.[115]
Thus most of our external sensors react only to rather rapid changes in conditions. However, we also have additional sensors that do not fade away, but keep responding to certain particular harmful conditions; see §§Alarms.
Now let’s apply the same idea—of a system that ‘mainly reacts to change’—to a brain with a tower of cognitive levels. This could help to explain some phenomena. For example, after you start a trip on a train, you’re aware of the clacking of wheels on the track—but (if that clacking is regular) then you will soon stop noticing this. Perhaps your A-Brain is still processing it, but your B-brain has stopped reacting to it. It will be much the same for the visual scenes; when the train enters a forest, you’ll start seeing trees—but soon you’ll start to ignore them. What could cause such meanings to fade?
It’s much the same with repeated words; if someone says ‘rabbit’ one hundred times, while trying to focus on what that word means, then that meaning will shortly disappear—or be replaced by some other one. And similarly the same thing happens when you listen to popular music: you’ll often hear dozens of nearly identical measures, but the details of these soon fade away and you no longer pay any attention to them. Why don’t we object to that repetitiousness?
This could be partly because we tend to interpret such ‘narratives’ in terms of how situations change on successively larger scales of time. In the case of most music, this structure is clear: we begin by grouping separate notes into ‘measures’ of equal length, and we then group these into larger sections, until the whole composition is seen as a storylike structure.[116] We do this in vision and language, too—although with less repetitiousness— by grouping collections of smaller events into multiple levels of events, incidents, episodes, sections, and plots. However, we see most clearly in musical forms:
Feature-Detectors recognize pauses, notes, and various other aspects of sounds, such as harmony, tempo, and timbre, etc.
Measure-Takers group these into chunks. In music, composers make this easy for us by using measures of equal length; this makes it extremely easy to sense the differences between successive chunks.
Phrase- and Theme-Detectors then represent larger events and relationships like, ‘This theme goes down and then goes up, and ends with three short, separate notes.’
Then Section-Builders group these into larger-scale parts, such as, ‘these three similar episodes form a sequence that rises in pitch.’[117]
Finally, our Storytellers interpret these as similar to events in other realms—such as depicting a journey through space and time, or a skirmish among personalities. One special appeal of music is how effectively it can depict what we might call abstract emotional scripts—stories that seem to be about entities about whom we know nothing at all except that we can recognize their individual characteristics—e.g., this one is warm and affectionate, whereas that one is cold and insensitive. Then we empathize with how they feel as we interpret those phrases and themes as representing mental conditions like conflict, adventure, surprise, and dismay—as in, those horns are attacking the clarinets, but the strings are now trying to calm them down.
115
See Peter Kaiser’s www.yorku.ca/eye/disapear.htm. [Also, see §§Change-Blindness] However, there are some signals that do not ‘fade away.’ Because we also have some additional sensors that evolved to keep responding to certain particular harmful conditions. [See §§Alarms.]
116
Roger Schank has conjectured that this may be one of our principal ways to learn and remember—in
117
There are more details about this in my essay at /web.media.mit.edu/~minsky/papers/MusicMindMeaning.html