9.2.1. Default values and typicality

If we look more closely at the examples considered thus far, we can see that al­though in each case the term activates a frame, the properties activated are not of the same kind. It is here that the distinction between typical and essential prop­erties proves so productive. There are restaurants without menus or without wait­ers serving at the tables, but a restaurant where you could never eat would not be a restaurant. In other words, the slots of a frame, to adopt the terminology of artificial intelligence, relate to values that have different roles in the representation of a given term; some are typical values while others are essential to the definition of the meaning. We saw in the last chapter that both essential and typical prop­erties can be included in frames, as in prototypes. Now, although each term acti­vates all the properties of its frame or typical occurrence, not all of them can be

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considered default values: only typical components are default values of the repre­sentation.

We must remember that in the representation formulated by artificial intelli­gence, default values are those values assumed to be valid in the absence of explicit indications to the contrary, indications which can, however, always be introduced without thereby altering the meaning. What this means is that the fundamental characteristic of default values is their erasability. The typical restaurant has menus, waiters, and tablecloths, but each of these elements is a property that can be erased, leaving simply an atypical restaurant. Green lemons, albino tigers, and white whales are all non-typical exemplars of their respective categories, but they are still respectively lemons, tigers, and whales. Typical color is a default value. In the absence of information to the contrary, we assume that lemons are yellow, tigers have stripes, and whales are dark-colored, but these properties can always be erased in that they are typical but non-essential. On the other hand, the facts that lemons are fruits, tigers are feline, and whales are mammals are not default values but properties that cannot be erased without renegotiating the meaning of the terms. Something similar occurs in complex scenes activated by ptedicates: pran-zare necessarily implies that something edible is consumed. This feature cannot be erased and is not assumed by default.

The difference between the two types of property clearly emerges if we use the test of the adversative but, as we saw in chapter 6. But can block typical properties, the subject of probable inferences, but not essential ones, without producing a semantic anomaly requiring subsequent explanation or textual elabo­ration:

6. It is a tiger but it is albino.

(I.e., it does not have stripes which, by default value, we would typically expect it to have).

7. It is a lemon but it is green.

(I.e., it is not yellow as, by default value, we would typically expect).

8. * It is a tiger but it is not an animal.

9. * I had lunch but I did not eat.

Although typical values are not homogenous and it is possible to distinguish be­tween different forms of typicality according to the various lexical categories,¹² they are all characterized by their erasability—typical properties are the set of default values of a frame or prototypical scene. Differentiating between typical properties and essential properties is thetefore important from the point of view of comprehension, in that we can then specify the components of a semantic frame that are operating as default values, that is, as a probable but not certain inferential base.

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