W e have now to dig beneath the assumptions that are tacitly made by the Critical Scientific Theory, and to discover their precise meaning and value. In expound­ ing it we talked of a number of people all “ looking at the same penny.” We assumed that there is a certain place “ seen” by all the observers, and that in this place there is a round physical object. We have now to ask what is meant by a common place; what is meant by a physical object occupying that place ; and what is meant by calling that object round. We shall find that all these questions, which seem so childishly simple, present great difficulties, and can only be answered in highly Pickwickian senses. They seem easy, because we habitually confine ourselves to cases, which are indeed of frequent occurrence, and are of practical interest, but which really owe their simplicity to the existence of specially simple conditions. These conditions are not always fulfilled, and then difficulties arise. This happens, for instance, with mirror images which turn up in places where nothing relevant is going on. As a rule, we simply ignore these “ wild” sensa; but we shall find that the only way to deal fairly with all the facts is to base our theory on them, and to

regard “ tam e” sensa as owing their tameness to the fulfilment of certain special simplifying conditions.