Gödel, Escher, Bach An Eternal Golden Braid by Douglas R. Hofstadter

The Prototype Principle

The list above seems to be a hierarchy of generality-the top being a very
broad conceptual category, the bottom some very humble particular thing
located in space and time. However. the idea that a "class" must always be
enormously broad and abstract is far too limited. The reason is that our
thought makes use of an ingenious principle, which might be called the
prototype principle:

The most specific event can serve as a general example

of a class of events.

Everyone knows that specific events have a vividness which imprints them

so strongly on the memory that they can later be used as models for other

events which are like them in some way. Thus in each specific event, there is

the germ of a whole class of similar events. This idea that there is generality

in the specific is of far-reaching importance.

Now it is natural to ask: Do the symbols in the brain represent classes,

or instances? Are there certain symbols which represent only classes, while

other symbols represent only instances? Or can a single symbol serve duty

either as a class symbol or instance symbol, depending which parts of it are

activated? The latter theory seems appealing; one might think that a "light"

activation of a symbol might represent a class, and that a deeper, or more

complex, activation would contain more detailed internal neural firing

patterns, and hence would represent an instance. But on second thought,

this is crazy: it would imply, for example, that by activating the symbol for

"publication" in a sufficiently complex way, you would get the very complex

symbol which represents a specific newspaper burning in my fireplace. And

every other possible manifestation of every other piece of printed matter

would be represented internally by some manner of activating the single

symbol for "publication". That seems much too heavy a burden to place on

the single symbol "publication". One must conclude, therefore, that in-

stance symbols can exist side by side with class symbols, and are not just

modes of activation of the latter.

The Splitting-off of Instances from Classes

On the other hand, instance symbols often inherit many of their properties

from the classes to which those instances belong. If I tell you I went to see a

movie, you will begin "minting" a fresh new instance symbol for that

particular movie; but in the absence of more information, the new instance

symbol will have to lean rather heavily on your pre-existing class symbol for

"movie". Unconsciously, you will rely on a host of presuppositions about

that movie-for example, that it lasted between one and three hours, that it

was shown in a local theater, that it told a story about some people, and so

on. These are built into the class symbol as expected links to other symbols

(i.e., potential triggering relations), and are called default options. In any

(^352) Brains and Thoughts