When do objects form a whole? Pre-theoretically we are inclined to think that sometimes objects are parts of a larger object-pages are parts of a book, fingers are parts of a hand, and also parts of people. Other sets of objects we are somewhat inclined to think do not form a whole-your fingers and my palm do not make up an object in the way that my fingers and my palm do. Some dust in the Andromeda galaxy and some rock from the centre of Mt Everest do not present themselves as part of a whole in the way that, for example, the cells of an individual redwood tree do.So much for how things seem on first blush. We can try to improve our grasp of which things make up objects and which do not in various ways-we can assemble a range of judgements about what is part of what and see if there are interesting underlying generalisations, we can see how questions of what is part of what fit into other questions we are concerned about (what a person or an artifact is, for example, or theories of creation and destruction). We can take into account unexpected empirical discoveries (when we discovered the atomic nature of objects like trees or rocks or ourselves, with atoms being separated from each other by many atom's diameters, we became less inclined to think objects must be in contact in order for them to make up a whole). We can think about how to apply the concepts of part and whole outside the main areas they were initially applied-we can see what happens when we try to apply them to space, or time, or sets, or numbers, or meanings, or events. And we can take into account theoretical virtues such as simplicity and unificatory power, preferring simple powerful theories to gerrymandered, ad-hoc-seeming generalisations or to mere collections of data points. There may be other channels of investigation as well. In short, we can do metaphysics. 1