2 THE PRESIDENTIAL ADDRESS 
And now I must turn to the subject on which I have specially 
undertaken to speak—-the new world-picture presented to us by 
modern physics. It is a full half-century since this chair was last 
occupied by a theoretical physicist in the person of the late Lord 
Rayleigh. In that interval the main edifice of science has grown 
almost beyond recognition, increasing in extent, dignity and beauty, 
as whole armies of labourers have patiently added wing after wing, 
story upon story, and pinnacle to pinnacle. Yet the theoretical 
physicist must admit that his own department looks like nothing so 
much as a building which has been brought down in ruins by a 
succession of earthquake shocks. 
The earthquake shocks were, of course, new facts of observation, 
and the building fell because it was not built on the solid rock of 
ascertained fact, but on the ever-shifting sands of conjecture and 
speculation. Indeed it was little more than a museum of models, 
which had accumulated because the old-fashioned physicist had a 
passion for trying to liken the ingredients of Nature to familiar 
objects such as billiard-balls, jellies and spinning tops. While he 
believed and proclaimed that Nature had existed and gone her way 
for countless aeons before man came to spy on her, he assumed that 
the latest newcomer on the scene, the mind which could never get 
outside itself and its own sensations, would find things within its 
limited experience to explain what had existed from all eternity. 
It was expecting too much of Nature, as the ruin of our building 
has shown. She is not so accommodating as this to the limita- 
tions of the human mind ; her truths can only be made compre- 
hensible in the form of parables, 
Yet no parable can remain true throughout its whole range to the 
facts it is trying to explain. Somewhere or other it must be too wide 
or too narrow, so that ‘ the truth, the whole truth, and nothing but 
the truth’ is not to be conveyed by parables. The fundamental 
mistake of the old-fashioned physicist was that he failed to distinguish 
between the half-truths of parables and the literal truth. 
Perhaps his mistake was pardonable, perhaps it was even natural. 
Modern psychologists make great use of what they describe as ‘ word- 
association.’ ‘They shoot a word at you, and ask you to reply im- 
mediately with the first idea it evokes in your uncontrolled mind. 
If the psychologist says ‘ wave,’ the boy-scout will probably say 
‘flag,’ while the sailor may say ‘sea,’ the musician ‘ sound,’ the 
engineer “ compression,’ and the mathematician ‘ sine’ or ‘ cosine.’ 
Now the crux of the situation is that the number of people who will 
give this last response is very small. Our remote ancestors did not 
survive in the struggle for existence by pondering over sines and 
cosines, but by devising ways of killing other animals without being 
killed themselves. As a consequence, the brains we have inherited 
