564 EVENING DISCOURSES. 
of recession of the nebulz would suggest. There is, I think, every reason to hope 
that within a very short time we shall know the truth about this puzzle, and, whatever 
the solution may be, there seems to be a considerable chance that it may provide us 
with a clue, perhaps even with a key, to the structure of the universe as a whole. 
Until quite recently, the scientist, like the ordinary man, accepted the fundamental 
ingredients of our experience—space, time, matter and energy—more or less at their 
face value. The most obvious and superficial interpretation suggested by everyday 
experience was assumed to correspond fairly closely to ultimate reality. The theory 
of relativity has shown that we were utterly wrong about space and time, and we 
are beginning to suspect that we are still just about as far wrong about matter and 
energy. The concept of an expanding universe may prove after all to be a false scent, 
and the truth may lie in some other direction. In either case the observed phenomena 
must mean something, and their true interpretation, when it is found, may carry us a 
step on towards the solution of the greatest mysteries of the external world—the 
nature of space, and of time, matter and energy, and of the combination of all these 
which constitutes the physical universe. 
(Published in Nature, Nov. 14, 1931.) 
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