With certain explanations and limitations laws I and II apply to many types of objects. In fact it requires an effort to realise that there are cases to which they do not apply. They have been stated above in the most formal manner to exhibit the fact that, when they do apply, they are empirical laws of nature and not à priori logical truths. 55. Components of Objects. 55.1 The concept of a 'component' of a main object is difficult to make precise. A component of an object is another distinct object ′ such that (i) whenever is situated in an event , there is an event ′, which is either itself or a part of , in which ′ is situated, and (ii) ′ may also be situated in an event ″ which is not a situation of or any part of a situation of . Thus a component is necessary to the main object, but the main object is not necessary to the component. For example, a certain note may be necessary for a certain tune, but the note can be sounded without the tune. The main object requires its component, but the component does not require the main object. But this general idea of a component is not of great importance apart from further specialisation. There are many such specialisations; but in science there are three which are of peculiar importance, namely, 'concurrent components,' 'extensive components' and 'causal components.' 55.2 An object ′ is a 'concurrent' component of an object when it is a component of , and if be any situation of , there is an event ′ which is part of and is such that (i) it is a situation of ′ and (ii) it is cut in a slice which is a situation of ′ by any duration which cuts in a slice which is a situation of . Thus a concurrent component lasts concurrently with the main object in any time-system. CHAPTER XV MATERIAL OBJECTS 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 () form a sort of 'material' objects when (i) the objects of the set are all uniform, (ii) not more than one member of can be located in any volume, (iii) no member of can be located in two volumes of the same moment, (iv) if and be two members of respectively located in non-overlapping volumes in the same moment, then any pair of situations of and respectively are separated events, (v) if be a member of situated in an event , and located in the volume which is a section of , and be any volume which is a portion of , then there is a member of which is located in and is a concurrent component of .