THE PRESIDENTIAL ADDRESS 13 
than the question. Our question as to what determines which 
photons get through is, I think, of a similar kind, andif Nature seems 
to answer ‘ Mere chance,’ she is merely answering us according to 
our folly. A parable which replaces radiation by identifiable photons 
can find nothing but the finger of fate to separate the sheep from 
the goats. But the finger of fate, like the photons themselves, 
is mere pictorial detail. As soon as we abandon our picture of 
radiation as a shower of photons, there is no chance but complete 
determinism in its flow. And the same is, I think, true when the 
particle-photons are replaced by particle-electrons. 
We know that every electric current must transfer electricity 
by complete electron-units, but this does not entitle us to replace 
an electric current by a shower of identifiable electron-particles. 
Indeed the exclusion-principle of Pauli, whichis in full agreement with 
observation, definitely forbids our doing so. When the red and white 
balls collide on a billiard table, red may go to the right and white 
to the left. ‘The collision of two electrons A and B is governed by 
similar laws of energy and momentum, so that we might expect 
to be able to say that A goes to the right, and B to the left or vice- 
versa. Actually we must say no such thing, because we have no 
right to identify the two electrons which emerge from the collision 
with the two that went in. Itis.as though A and B had temporarily 
combined into a single drop of electric fluid, which had subsequently 
broken up into two new electrons, C, D. We can only say that 
after the collision C will go to the right, and D to the left. If we are 
asked which way A will go, the true answer is that by then A will 
no longer exist. ‘The superficial answer is that it is a pure toss-up. 
But the toss-up is not in nature, but in our own minds ; it is an even 
chance whether we choose to identify C with A or with B. 
Thus the indeterminism of the particle-picture seems to reside 
in our own minds rather thanin nature. In any case this picture 
is imperfect, since it fails to represent the facts of observation. ‘The 
wave-picture, which observation confirms in every known experi- 
ment, exhibits a complete determinism. 
Again we may begin to feel that the new physics is little better 
than the old—that it has merely replaced one determinism by 
another. It has; but there is all the difference in the world between 
the two determinisms. For in the old physics the perceiving mind 
was a spectator ; in the new itis an actor. Nature no longer forms 
a closed system detached from the perceiving mind ; the perceiver 
and perceived are interacting parts of a single system. ‘The nature 
depicted by the wave-picture in some way embraces our minds 
as well as inanimate matter. Things still change solely as they 
are compelled, but it no longer seems impossible that part of the 
compulsion may originate in our own minds. 
