lawnmowers & models

[Avery Andrews 93107.1206]
  (Bill Powers (930925.0800)

More on lawnmower repair.

>One of them is the answer to the question,
>"Why did I take the top off the lawnmower?" The answer could not
>have been "to grasp the frayed end of the cord and stuff it
>through the hole," because you couldn't see either where the end
>of the cord was, or that it was frayed, or what "stuffing" would

What I actually knew was that the children had told me that the handle
had come off, & given me the handle and little metal bar that goes
through it, and cord wasn't to be seen. But I knew where it was,
partly from common-sense, and partly because the same thing had
happened a few years ago while I was watching someone else try to
start the mower. I couldn't remember much of the details of what
I did the last time (nor whether I figured it out then, or whether
somebody told me what to do). So first time around, I think that
what you know is that the cord is in their somewhere, and that you're
going to have to take something off to find out what's going on.
But by the time you come to have to cope with this kind of problem for
the first time, you've coped with many similar ones, and had
considerable guidance from other people on how to deal with them.

>Before the top is off, all you have is an imagined model
>of what might be under the cover (assuming you've never done it
>or seen it done before). This model tells you that if you try to
>reach the cord without taking the top off, the top will get in
>the way. You don't need to try this in actuality -- reach for the
>imagined position of the cord and try to grasp it.

So I doubt that it's necessary to even imagine trying to grasp the end
of the cord, and noticing that it didn't work (even if the housing was
made of transparent plastic, so that you could actually see the end of
the cord). I suspect that a rule along the lines of `nothing accessible
to work on, so take something off' is being followed. But supposing that
imagination were involved here, how would it actually work? What
comes to my mind is Marr-style 3d model manipulation, with collision
detection circuitry - a rather complicated gadget that evolution
bestowed upon our simian forbears. To see how people coped with this
kind of problem for the first time, maybe you'd have to give children
things made out of some kind of Lego with lots of transparent parts -
I can certainly imagine a 4 year old (or a drunk, desparate, panicking
or otherwise demented adult scrabbling at a transparent housing as if
they could get at the bits inside that way).

>Then your model tells you that the top is held on in some way (if
>the casing is all in one piece you'd have to hacksaw it off,
>which doesn't seem a reasonable requirement -- how, you wonder,
>would they have assembled the thing?). Anyway, this leads you to
>look for something to unfasten. A bolt head or slotted screw head
>would fill the bill. So you search for something in that

>You can't actually see the part of the bolt or screw that is
>under the cover, but you can imagine it. Your model includes the
>properties of bolts and screws, namely the fact that twisting is
>involved; also your model contains the correct direction of twist
>for unfastening. So you find the unfastening tool and use it, and
>remove the cover.

I strongly doubt the role of `models' here. The somewhat experienced
person expects that it will be possible to get inside the thing, and
that the way do to this is to take something off, and that that
something will be attached by fasteners of various familiar kinds,
and knows what to do with these kinds of fasteners. Where models
come in is when the standard methods don't work. For bolts and screws,
for example, the top-level reference perception is for them to be
`out', which is *normally* achieved a perception that they are
coming out due to being twisted. There are standard means of twisting
them, but sometimes nonstandard ones need to be used instead --
I've wrecked the heads of screws by incompetent use of a screwdriver,
but gotten them out anyway by grabbing the head with snap-lock pliers
and twisting. But suppose you twist, but there is no perception
of coming out. Then you know you have a problem, and a model
might be employed to solve it. Maybe there's a nut that's not attached
to anything but the bolt, so you're turning the bolt and nut together
but not making any progress. So you might use your model, and try
pulling the bolt up hard at the same time as you turn it, so the
nut will be held against the underside of the cover, and prevented from
turning. Or you might try sticking a hacksaw blade in somewhere
and trying to cut throught the bolt. I see no need for `models' until
you do something intelligent, as opposed to just imitating what you've
seen people do, or trying things you've been told about.

The story about the fasteners illustrates the role of `programs' that
consists of perceptual targets with *suggestions* about sequences
and combinations of other perceptual targets that will make it easier
to attain them, but no guarantees of success, and concomitantly no
requirement to stick to the program. E.g.

   goal: fastener out.
    suggested subgoals: fastener turning and coming out

Note that this program is the same for bolts and for screws, but these
differ in the standard method for perceiving the fastener as turning:

  goal: fastener turning:
    subgoal if screw: insert screwdriver into slot & turn
    subgoal if bolt: fit pliers around head and rotate

I suspect that very few people who know how to fix things had to figured
out how to do these things for themselves. Mostly, I think you learn
by watching, so that one thing that people probably have as part of
their innate equipment is some gadetry to convert perceptions of
other people doing things into perceptions of what it would be like to
be doing those things yourself.