I am not the least concerned to defend this theory. It has been much derided, and is certainly open to attack. But something like it seems to be an inevitable stage in the development of modern views of nature. The whole effort of physical science is to discover the material or non-psychical facts which shall, among other things, account for our psychical experiences. It is true that there are men of science, as well as philosophers, who regard all such constructions as purely arbitrary—mere labour-saving devices which have nothing to do with reality. But though I shall have something to say about these theories in my next course of lectures, for the present I need only observe that they do not represent ordinary scientific opinion, either as it is, or as it has ever been. Science thinks, rightly or wrongly, that she is concerned with a real world, which persists independently of our experience: she has never assented to the doctrine that the object of her patient investigations is no more than a well-contrived invention for enabling us to foretell, and perhaps to modify, the course of our personal feelings. But then, if science is right, we are committed to a division between the contents of immediate experience and its causes, which showed itself dimly and tentatively in the distinction between the secondary and the primary qualities of matter, but has become deeper and more impassable with every advance in physics and physiology. It was possible to maintain (though, I admit, not very easy) that, while the secondary qualities of matter are due to the action of the primary qualities on our organs of perception, the primary qualities themselves are, nevertheless, the objects of direct experience. The fact, for example, that colour is no more than a sensation need not preclude us from perceiving the material qualities which, like shape, or motion, or mass, are the external and independent causes to which the sensation is due. I do not say that this view was ever explicitly entertained—nor does it signify. For, if we accept the teaching of science, it can, I suppose, be entertained no more. The physical causes of perception are inferred, but not perceived. The real material world has been driven by the growth of knowledge further and further into the realm of the unseen, and now lies completely hidden from direct experience behind the impenetrable screen of its own effects. IV For consider what the causal process of perception really is if we trace it from the observed to the observer—if we follow the main strands in the complex lines of communication through which the object seen reveals itself to the man who sees it.