 A glass has certain dispositions, for example the disposition to shatter when struck. But what is this disposition? It seems on the one hand to be a perfectly real property, a genuine respect of similarity common to glasses, china cups, and anything else fragile. Yet on the other hand, the glass’s disposition seems mysterious, ‘ethereal’ (as Goodman (1954) put it) in a way that, say, its size and shape are not. For its disposition, it seems, has to do only with its possibly shattering in certain conditions. In general, it seems that nothing about the actual behaviour of an object is ever necessary for it to have the dispositions it has. Many objects differ from one another with respect to their dispositions in virtue of their merely possible behaviours, and this is a mysterious way for objects to differ.