IT seems likely that even those readers who are broadly in agreement with the thesis here presented will have moments of doubt when they feel that it is altogether too full of holes. For they will continu ally come across situations which look very much like exceptions when contemplated by a mind brought up in traditional habits. For example, in the chapter on the Size of a Unit I have suggested that the question whether X is one thing or several things depends in part upon the pur pose for which X is being paid attention to. What, then, about the (at present) unsettled question whether a single gene or many genes is or are the causal factor in inheritance of haemophilia? What also about the now settled question whether vitamin B is a single substance or a ‘complex’? In order to answer these questions we surely do not have to concern ourselves in any way with purpose. . . . Again, what about the many fields of enquiry-gardening, making love, making locomotives, for example-where we obviously never need to determine contexts at all?