CHAPTER XVIII 
THE INTERPRETATION OF THE QUANTUM 
From the point of view of the physicist, a theory of matter is a 
policy rather than a creed ; its object is to connect or to co-ordinate 
apparently diverse phenomena, and above all to suggest, stimulate, 
and direct experiment. 
J. J. Thomson, The Corpuscular Theory of Matter, p. i, 1907 
Grant I have mastered learning’s crabbed text, 
Still there’s the comment. 
Let me know all! Prate not of most or least, 
Painful or easy ! 
Even to the crumbs Pd fain eat up the feast, 
Ay, nor feel queasy. 
Robert Browning, A Grammarian’s Funeral 
1. The Meaning of “Explanation” in Physics 
I N attempting to review the suggestions that have been made 
as to the interpretation of the quantum, it may be well to 
consider at the outset what exactly is implied in physical 
science by such a term as “ explanation ” or “ interpretation.” 
In ordinary language we speak of explanation when some strange 
or unfamiliar phenomenon is expressed in terms of processes or 
events which are familiar to us, and are called well-known, though 
we may be, and often are, in ignorance of their true character. 
The concept of “ explanation ” must pass through a process 
of evolution with the progress of knowledge. “ To explain a 
phenomenon is, for primitive man, to interpret it anthropo- 
morphically by a supernatural agent endowed with psychological 
life in his own image ; for a scholastic it is to explain it by 
ultimate causes ; for Bacon to explain it by efficient causes ; for 
Maxwell it is to deduce it from the principles of mechanics; for 
Gibbs and Boltzmann it is to account for it by the calculus of 
probabilities, by starting from a system of elements subject to 
given conditions.” * 
In modern scientific work we are said to explain phenomena 
when we show that they can be expressed as examples of certain 
general “ laws ” which have come to be recognized as holding in 
* Rougier, Philosophy and the New Physics, p. 146 (Routledge). 
17 243