From these questions concerning the physical world in itself, we were led to others concerning the causation of our perceptions, which are the data upon which our scientific knowledge of physics is based. We saw that a long causal chain always intervenes between an external event and the event in us which we regard as perception of the external event. We cannot therefore suppose that the external event is exactly what we see or hear; it can, at best, resemble the percept only in certain structural respects. This fact has caused considerable confusion in philosophy, partly because philosophers tried to think better of perception than it deserves, partly because they failed to have clear ideas on the subject of space. It is customary to treat space as a characteristic of matter as opposed to mind, but this is only true of _physical_ space. There is also _perceptual_ space, which is that in which what we know immediately through the senses is situated. This space cannot be identified with that of physics. From the standpoint of physical space, all our percepts are in our heads; but in perceptual space our percept of our hand is outside our percept of our head. The failure to keep physical and perceptual space distinct has been a source of great confusion in philosophy. In Part III we resumed the study of man, but now as he appears to himself, not only as he is known to an external observer. We decided, contrary to the view of the behaviourists, that there are important facts which cannot be known except when the observer and observed are the same person. The datum in perception, we decided, is a private fact which can only be known directly to the percipient; it is a datum for physics and psychology equally, and must be regarded as both physical and mental. We decided later that there are inductive grounds, giving probability but not certainty, in favour of the view that perceptions are causally connected with events which the percipient does not experience, which may belong only to the physical world.