four-dimensional world (for logical, as distinct from experimental, reasons) can only be compared to a man who doubts the reality of the penny, and prefers to regard one of its innumerable appearances as the real object.
Physical reality is the synthesis of all possible physical aspects of nature. An illustration may be taken from the phenomena of radiant-energy, or light. In a very large number of phenomena the light coming from an atom appears to be a series of spreading waves, extending so as to be capable of filling the largest telescope yet made. In many other phenomena the light coming from an atom appears to remain a minute bundle of energy, all of which can enter and blow up a single atom. There may be some illusion in these experimental deductions; but if not, it must be admitted that the physical reality corresponding to light must be some synthesis comprehending both these appearances. How to make this synthesis has hitherto baffled conception. But the lesson is that a vast number of appearances may be combined into one consistent whole—perhaps all appearances that are directly perceived by terrestrial observers—and yet the result may still be only an appearance. Reality is only obtained when all conceivable points of view have been combined.
That is why it has been necessary to give up the reality of the everyday world of three dimensions. Until recently it comprised all the possible appearances that had been considered. But now it has been discovered that there are new points of view with new appearances; and the reality must contain them all. It is by bringing in all these new points of view that we have been able to learn the nature of the real world of physics.
Let us briefly recapitulate the steps of our synthesis. We found one step already accomplished. The immediate perception of the world with one eye is a two-dimensional appearance. But we have two eyes, and these combine the appearances of the world as seen from two positions; in some mysterious way the brain makes the synthesis by suggesting solid relief, and we obtain the familiar appearance of a three-dimensional world. This suffices for all possible positions of the observer within the parts of space hitherto explored. The next step was to combine
