254 THE QUANTUM [xviii. 5 
intellectual satisfaction, and yet it would be premature to come 
to a definite decision as to the form the theory will ultimately 
assume. 
In the foregoing pages some of the difficulties still to be over 
come have been pointed out, and a number of suggestions have 
been made as to possible interpretations of the quantum. But 
the scientific student cannot be satisfied with mere speculations, 
for, in the words of Max Planck: “ Our aspiration after a uniform 
theory of nature, on a mechanical basis or otherwise . . . can 
never be permanently repressed.” We may with confidence 
anticipate the day when the constitution of the proton and the 
electron, the structure of the electromagnetic field, and the nature 
of light shall stand more clearly revealed through the elucidation 
of the principles of the quantum theory. 
In the rapid developments which are taking place at the 
present time in the field of experimental and theoretical physics 
we may trace a parallel to the growth of philosophical ideas as 
recorded by Benedetto Croce. 
“ To-day I observe in my own case the impossibility of resting 
upon the results of past thought: I see a new crop of problems 
springing up in a field from which I have but now reaped a 
harvest of solutions ; I find myself calling in question the con 
clusions to which I have previously come ; and these facts, which 
appear in every part of philosophy as I handle and rehandle it, 
force me to recognize that truth will not let itself be tied fast for 
ever. They teach me modesty towards my present thoughts, 
which to-morrow will appear deficient and in need of correction, 
and indulgence towards my self of yesterday or the past, whose 
thoughts, however inadequate in the eyes of my present self, 
yet contained some real element of truth ; and this modesty and 
indulgence pass into a sense of piety towards thinkers of the 
past, whom now I am careful not to blame, as once I blamed 
them, for their inability to do what no man, however great, can 
do—to close the eternal gates of truth, to fix into eternity the 
fleeting moment.”