Steven Pinker -- How the Mind Works - Hampshire High Italian ...

Thinking Machines 115tions. Rather than symbolizing an entity as an arbitrary pattern in a stringof bits, we represented it as a pattern in a layer of units, each standingfor one of the entity's properties. An immediate problem is that there isno longer a way to tell apart two individuals with identical properties.They are represented in one and the same way, and the system is blind tothe fact that they are not the same hunk of matter. We have lost theindividual: we can represent vegetableness or horsehood, but not a particularvegetable or a particular horse. Whatever the system learns aboutone horse melds into what it knows about another, identical one. Andthere is no natural way to represent two horses. Making the horsey nodestwice as active won't do it, because that is indistinguishable from beingtwice as confident that the properties of a horse are present or fromthinking that the properties of a horse are present to twice the degree.It is easy to confuse the relationship between a class and a subclass,such as "animal" and "horse" (which a network handles easily), with therelationship between a subclass and an individual, such as "horse" and"Mr. Ed." The two relationships are, to be sure, similar in one way. Inboth, any property of the higher entity is inherited by the lower entity. Ifanimals breathe, and horses are animals, then horses breathe; if horseshave hooves, and Mr. Ed is a horse, then Mr. Ed has hooves. This canlure a modeler into treating an individual as a very, very specific subclass,using some slight difference between the two entities—a freckle unitthat is on for one individual but off for the other—to distinguish neardoppelgangers.Like many connectionist proposals, the idea dates back to Britishassociationism. Berkeley wrote, "Take away the sensations of softness,moisture, redness, tartness, and you take away the cherry, since it isnot a being distinct from sensations. A cherry, I say, is nothing but acongeries of sensible impressions." But Berkeley's suggestion never didwork. Your knowledge of the properties of two objects can be identicaland still you can know they are distinct. Imagine a room with two identicalchairs. Someone comes in and switches them around. Is the roomthe same as or different from before? Obviously, everyone understandsthat it is different. But you know of no feature that distinguishes onechair from the other—except that you can think of one as Chair NumberOne and the other as Chair Number Two. We are back to arbitrarylabels for memory slots, as in the despised digital computer! The samepoint underlies a joke from the comedian Stephen Wright: "While Iwas gone, someone stole everything in my apartment and replaced it