4.3 Scientific analyses of powers or dispositions

Consider how science might analyze the fragility of a glass vase, namely the disposition to break after small external pressures. The very first analysis would be to treat the vase as a whole with mass, shape, rigidity and fragility. The fragility is then a property of the vase. The vase is therefore an object with specific dispositional properties and, as well, with a shape and orientation. The second analysis would be to consider that the vase is made of glass, where the glass is a continuous solid with various mass, elastic and fracture properties. A computer finite-element model of the vase might then explain its fragility in terms of the stress and fracture properties of the constituent material. In this case, the glass is the dispositional material, to be arranged in the shape of the vase and thereby to explain the properties of the vase. A third level of analysis might be a molecular simulation, where elasticity and fractures are properties derived from the strengths of interaction potentials between molecules. Now, the molecules are the objects constituted by those interaction potentials, which are dispositions, and they are arranged to make macroscopic glass-material. And so on: a fourth level may consider the potentials between individual electrons and nuclei, where now those electrons and nuclei are constituted by mass, charge, spin, magnetic moments, etc.: all dispositional properties. Surely quantum mechanics is also needed, which introduces its own set of probabilistic dispositions (propensities).

We see that at each stage of microscopic analysis, the presented objects are diagnosed as structural forms of some more fundamental disposition. Whether the stages reach the most fundamental level is not the issue here. Rather, at each level, the result of the analysis is to attribute existence to some ‘stuff’ with some causal powers held essentially. First, the vase as a whole was the existing stuff; in the second analysis, the glass with stress-strain powers is taken as the stuff of the vase; later it is electrons, etc., with electric charges; and the final stage listed here has electrons with propensities to emit or absorb virtual photons.

The attribution of dispositions in all of the above cases is according to the following logical template:

“Object S has the disposition P to do action A” is equivalent to “if S is in some circumstance C, C depending on P and the character of A, then there will be a non-zero likelihood of S doing A”.

Here, the ‘action A’ can either be a change in S itself or an interaction with other objects. The suitable ‘circumstance C’ is usually defined by multiple spatial relations to other objects and will be different for different dispositions and for different actions. The circumstance C is said to depend only on the ‘character’ of the action, and not on the action itself, because possibly, if the disposition is never manifested, there may exist no such action at any time in the past, present or future. Finally, the phrase ‘non-zero likelihood’ is designed to be sufficiently general to allow both sure-fire dispositions and probabilistic propensities. It has the consequence that if the probability of an event (while varying with time) touches zero, then there is no propensity at that particular time, but this is surely a reasonable feature.

In all cases above, we never avoided dispositional properties again in the explanation of the first disposition to be explained. Dispositionality (of some kind) never seems to go away because of physical explanations! We seem to have some kind of ‘dispositional essentialism’, a claim which asserts that that each object has some properties that are inherently dispositional, and that these include the causal base properties that enter into scientific laws. Since Ellis and Lierse (1994), many have supported this argument. Similar views are advocated in Bird (2005a), Bird (2007), Cartwright (1983), Chakravartty (2003), Elder (1994), Ellis (2000), Ellis (2001) Fetzer (1977), Harré and Madden (1975), Molnar (2003), Mumford (1995), Mumford (1998), Shoemaker (1984), Swoyer (1982), and Thompson (1988).

An opposing view is given by Ryle (1949), who sees dispositions as merely ‘inference tickets’ or ‘promises’, even when science gives perfectly good explanations! There is another tension between accounts of nature based on dispositions and accounts based on laws. Armstrong (1969) and Katzav (2004), for example, see dispositions as derived from universal laws combined with non-dispositional properties, but, as Bird (2005b) points out, this does not solve all the philosophical problems. Either Armstrong must concede that some properties are not categorical but instead have essential powers, or he is faced with a regress. I adopt the above type of dispositional essentialism as the basis for further developments in this book, claiming that every object has some dispositional properties that enter into its causal relations.^4.4