Ideally, physical systems are causally connected structures that are causally isolated from other things. There is no good reason to believe that there are any physical systems that are really isolated in this way. But this does not matter, since we are usually able to abstract from the actual circumstances of things to consider how they would be in themselves, and so discuss their various properties. The problem of abstraction becomes more difficult the smaller or more diffuse the objects we are concerned with. For then, the identities of objects are less well defined. But, in this chapter we shall be concerned with the properties of ordinary middle-sized things, without regard to these difficulties. In the kinds of cases to be discussed, they hardly arise.

It will be argued here that there are two essentially different kinds of properties of physical systems. There are properties whose natures are dispositional, and ones whose natures are structural. The former are often grouped together as “causal powers”, and the latter as “categorical properties”, But I do not much like this way of classifying properties, because it suggests, wrongly in my view, that dispositionality is a sufficient condition for causality. The causal powers are just a proper subset of the dispositional properties, and will be discussed as such.

To facilitate discussion of these two kinds of properties, it will be helpful to introduce, at the outset, the concept of a dimension. Dimensions, as I shall explain presently, are respects in which things may be the same or different. The quantities, for example, are all dimensions.