Husserl's third Logical Investigation is called "On the theory of wholes and parts". It has probably received less attention from commentators than any of the other investigations, including the shorter fourth, which Husserl himself saw as an application of the ideas of the third to questions of grammar. The ideas put forward in the third investigation play a crucial role in Husserl's subsequent philosophy, and he was able to recommend them, even much later in his life, as offering the best way into his philosophy.² Although they did not perhaps present such an attractive clarion-call to research, they might, had Husserl's advice been followed, have made a much greater contribution to philosophical work than in fact they did. I should like to suggest that it is not too late to learn from the third investigation, and that, in a tidier form than they there receive, the ideas could become indispensable weapons in the conceptual armoury of the philosopher interested in ontology. This chapter has the more modest purpose of attempting to clarify and interpret what Husserl was trying to say, with a view to eventually offering a rigorous treatment of the most important notions, and I wish also briefly to suggest where such notions might prove important in ontology.