Place, U. T. (1994d). Sharpness: an interesting exception to the rule that dispositional properties require explanation in terms of their owner's microstructure [Conference presentation, presented to the Twentieth Annual Conference on the Philosophy of Science at the Inter University Centre, Dubrovnik, Croatia, 12th April 1994]. Inter University Centre, Dubrovnik.
[Abstract]The most common form of distinctively scientific causal explanation is an explanation of the dispositional properties shared by instances of a universal or kind. Such explanations typically invoke the structural properties of the property-bearer. In the majority of cases and in all cases where a specifically scientific explanation is required, what are invoked are features of the microstructure of the property-bearer which are not accessible to ordinary observation at the level of common sense. An interesting exception is the case of the sharpness of a knife or needle. Sharpness is a property and a concept with a number of unusual features. Most property-concepts are either purely dispositional, as in the case of such things as the brittleness of glass, the flexibility of rubber or the magnetic properties of an iron bar, or they are structural properties, such as the external shape and internal arrangement of an object. Sharpness, by contrast, is a property with two aspects, a purely dispositional aspect, the property-bearer's propensity to cut or pierce, and a structural aspect, the fineness and hardness of its edge or point. However, the relation between these two aspects is a causal relation between "distinct existences", not a relation of identity. The dispositional property, aptness to cut or pierce, depends on and is explained by the structural properties, the fineness and hardness of the edge or point. In this it differs from most other dispositional properties. For in this case, the structural properties on which the dispositional property depends are features of the macrostructure rather than the microstructure of the property-bearer. They are thus available to common observation by the man- or woman-in-the-street in a way that the microstructural properties on which most dispositional properties depend are not. Hence the absorption of both cause and its effect into a single common-sense concept. Causal relations and the causal explanations which invoke them have two components: (a) a categorical component, some kind of contact or proximity between the causal agent and the causal patient, and (b) a dispositional component which provides the "cement" which, in the explanation, takes the form of a 'covering law' and governs the interaction between the two. In this respect, the causal relation whereby aptness to cut or pierce is generated by the structural properties of fineness and hardness of edge or point is no exception. Of the two structural properties which stand as cause to the dispositional property as effect, one, the fineness of the edge or point, is categorical; the other, its hardness, is dispositional. From a philosophical standpoint the 'sharpness' example raises two interesting questions: (1) In what sense does the effect, the aptness to cut or pierce, constitute a "distinct existence" from its causes, the fineness and hardness of the edge or point, as Hume's principle requires? (2) What light, if any, is thrown by this example on the problem of the source of the dispositional properties of an elementary particle which has no microstructure (the 'charm' of the quark)?
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