The paper draws on the distinction, first expounded by Husserl, between formal logic and formal ontology. Formal logic concerns itself with meaning-structures; formal ontology with structures amongst objects and their parts. We show how, when formal-ontological considerations are brought into play, contemporary extensionalist theories of part and whole, and above all the mereology of Lesniewski, can be generalised to embrace not only relations between concrete objects and object-pieces, but also relations between what we shall call dependent parts or moments. A two-dimensional formal language is canvassed for the resultant ontological theory, a language which owes more to the tradition of Euler, Boole and Venn than to the quantifier-centred languages which have predominated amongst analytic philosophers since the time of Frege and Russell. Analytic philosophical arguments against moments (accidents, tropes, individual qualities), and against the entire project of a formal ontology, are considered and rejected. The paper concludes with a brief account of some applications of the theory presented.

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