# Dealing with Multiple Classifications

Erik Nygren
6.868 Paper 3
April 30, 1996

Much of our knowledge about the world can be structured in taxonomies. By knowing the characteristics of the superclass of an object, we can make the inferences about the characteristics of the object itself. However, the world is not strictly hierarchical. Many objects and concepts belong to more than one superclass. In many cases, the properties of superclasses conflict or have contradictory goals. Using a form of level bands called class bands, we can maintain multiple classifications and use them effectively.

We often find ourselves trying to classify things in strict hierarchies. Birds and dogs are a subset of animals which is, in turn, a subset of alive things. Hierarchical classifications like these are useful because more specific objects and classes of objects tend to inherit attributes from their super classes. When we see a new kind of bird which we've never seen before, we can assume that it probably flies because most birds fly. We can also assume that if we approach it, it will react to our presence, because most animals tend to do so. This sort of classification prevents us from having to relearn everything whenever we encounter a new instance of something. This scheme also allows us to generalize newly obtained knowledge. If we are bitten by a german shepherd, we'll be more careful the next time we encounter a golden retriever, even though we've never been bitten by one before.

It's clear how we deal with strict hierarchies, but the world isn't strictly hierarchical. A porcelain duck is both pottery and a bird. As a result of this lack of strict hierarchies, classes inherit attributes from multiple superclasses. We know the porcelain duck is breakable, but we can also pretend that it's a bird which can fly. In many cases, characteristics of two super classes conflict. How do we deal with these conflicts? If we tried to accept both characteristics, we would have too much information to contend with and we would get confused. If we applied the Principle of Non-Compromise and gave less weight to conflicting characteristics, we might lose valuable information. Going back to the porcelain duck example, a bird has feathers but pottery has a smooth, hard surface. Clearly, the surface is hard and smooth, although we could imagine that it had feathers. However, it's hard to think about the bird having feathers and a hard surface at the same time.

One possibility is that level bands play a crucial role in maintaining and using multiple classifications. In addition to the traditional vertical levels of detail, another type of level banding called class banding might be used. When we look at something with multiple classifications, we tend to focus primarily on the classification which is most closely related to how we are interacting with it. The properties from other classifications will also be available, but they will be considerably less prominent. The class band is the classification which we are predominantly focusing on.

A stick is both a piece of wood and a pry bar. If we look at it as a stick, we will realize the brown color of the bark on it and we will realize that it's breakable. However, if we use the stick as a pry bar, we will hold it and treat it as such. Although we will keep in the back of our minds that it's also a breakable piece of wood, we will not always treat it as such, inevitably resulting in the stick getting broken. After having experience with this, the breakable property of the stick will become more prominent. If we create a representation for a wood pry bar, it will inherit most of the properties of pry bars (the active class band), but will also inherit the breakable property of wood sticks (which is a characteristic of sticks at the active "level of detail" band).

Class bands play an important part of how we interact with many things which have different classifications. As we showed, they can be used to focus on the aspects of a tool which are related to the application we're using it for. Class bands also play a part in how we interact with people. Someone may be both your thesis advisor and your tennis partner. Depending on which role they are in, the class band has a different focus and we'll interact with them in a different manner. As another example, an antique chair is both an antique and a chair. Depending our class band focus, either its "antiqueness" or its "sitability" will be prominent, although the other properties will be sufficiently active to influence how we interact with the chair.

Our ability to imagine the unreal and appreciate drama is influenced by class bands. Although an actor may be a friend of ours, when he's playing a role in a drama performance he is both our friend and the character he's playing. As we watch the production, we will hopefully focus the class band on "the character" rather than on "our friend." The role of class bands in imagination also allows children to play with the porcelain duck from the earlier example.

Class bands allow us to focus our attention on a single classification of an object or concept which has multiple classifications. However, they do not totally eliminate the influences of other classifications. This mechanism allows us to effectively deal with things which have multiple classifications.

Erik Nygren (nygren@mit.edu)