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Mereology/modal/modality/Simons: We need in the mereology modality - we need a definition "necessary part" - and a modal definition of "Sum" (because it is less intuitive than "organism" etc... - dependence: a) ontological dependence: an object cannot exist without another existing. - b) functional dependence: between characteristics of objects which form a whole.
Modal part/Simons: E.g. a class of counterparts might be considered as a whole with different modal components - modal parts: in various possible worlds.
Modality/Metaphysics/metaphysical/nature/necessity/Simons: a given atom could not have other particles - they are essential parts (components). - On the other hand: the given particles could have belonged to another atom. - It is not essential for them to be part of this atom. - "given".
Parts Oxford New York 1987