24.8 The essential unity of the object amid the spatial parts of its situation is more difficult to grasp. The derivation of space and time by the method of extensive analysis, as explained in Part III of this enquiry, exhibits the essential identity of extension in time and extension in space. Thus the reasons for denying temporal parts of an object are also reasons for denying to it spatial parts. Again, it is true that the leg of a chair occupies part of the space which is occupied by the chair. But in appealing to space we are appealing to relations between events. What we are saying is, that the situation of the leg of the chair is part of the situation of the chair. This fact only makes the leg to be part of the chair in a mediate derivative sense, by way of their relations to their situations. But the leg is one object with a recognisable permanence of association, and the chair is another, with recognisable permanence of association distinct from that of the leg, and their situations in all circumstances have certain definite relations to each other expressible [5] in temporal and spatial terms. 24.9 The second reason for the vagueness of physical objects is the impossibility of submitting the group of associations, forming the object, to any process of determination with a progressive approximation to precision. A physical object is one of those entities of ordinary experience which refuse to be pressed into the service of science by way of a progressive exactness of determination. Consider for example a definite object such as a certain woollen sock. It wears thin, but it remains the same object; it is darned, and remains the same object; finally after successive repairs no part of the original wool is left, but it is the same sock. The truth is that each time we affirm the self-identity of this object we are construing the group of associations, which we recognise, in a more and more attenuated sense. The object which is both the sock at the end and the sock at the beginning is a very attenuated complex type of permanence, which would not be what we meant by the sock merely at the beginning of its career or as perceived merely at the end of its career. By insisting on the continued identity of the sock, we are in fact continually juggling with what we mean by the sock, always retaining the most complete associations which we can trace through the whole continuous series of events forming the successive situations of the sock. The physical object 'works' perfectly for the ordinary usage of life, and is thus fully justified for that purpose in the eyes of the pragmatic philosopher.