Page:An Enquiry Concerning the Principles of Natural Knowledge.djvu/185



56. Material Objects. 56.1 A material object is essentially a material object of a certain definite sort; namely, we define sorts of material objects, which are sets of objects with certain definite peculiarities, and a material object is such because it is a member of one of these sorts. For example a piece of wood is a material object because it belongs to the class of wooden objects and because this class possesses the requisite peculiarities. Similarly a charge of electricity is a material object for an analogous reason.

The objects which compose a set (μ) form a sort of ‘material’ objects when (i) the objects of the set μ, are all uniform, (ii) not more than one member of μ can be located in any volume, (iii) no member of μ can be located in two volumes of the same moment, (iv) if O1 and O2 be two members of μ respectively located in non-overlapping volumes in the same moment, then any pair of situations of O1 and O2 respectively are separated events, (v) if O be a member of μ, situated in an event e, and located in the volume V which is a section of e, and V1 be any volume which is a portion of V, then there is a member of μ which is located in V1 and is a concurrent component of O.

56.2 If O be a material object of a certain sort and V be a volume in which O is located and V1 be a portion of V, then the material object of the same sort as O which is located in V1 is called an ‘extensive component’ of O.