246 THE QUANTUM [xviii. a 
describes simply, in its equations, the relations which connect 
the simultaneous variations of directly measurable physical 
quantities.” * This attitude of reserve may be prudent, but by 
excluding all explanatory theories it “ condemns us to an in 
tellectual asceticism.” The human mind cannot permanently 
remain satisfied with a mere catalogue of relations, and though 
doomed to disappointment in the search, strives to reach a more 
comprehensive explanation. 
There is no doubt profound truth in the statement of Sir Joseph 
Larmor: f “In ultimate logic any physical representation is in 
fact a mental construction or analogy, designed to relieve the 
mind from the intangible and elusive character of a complex of 
abstract relations.” But such a statement should rather encour 
age than prohibit attempts at physical representation. 
Sir Oliver Lodge has protested against “ a comparatively 
recent anti-scientific tendency to teach that we are not making 
an attempt to ascertain actual truth about the universe, but 
only to formulate propositions that are practically helpful and 
convenient.” % Is there then no ultimate scientific certainty ? 
Perhaps an answer to this question may be given by adopting 
words that have been used in another connection and saying that 
natural philosophy provides us with “ an image, in mind, of a 
reality previously existing.” The image may vary in different 
minds and in different generations, but it seems almost a necessity 
of human thought that there must be some reality behind our 
imaginings. “ Truth is in the ideal. The actual world is an 
approximation thereto. . . . We always aim at truth beyond 
the range of our experience or measuring achievements. Absolute 
truth may be an unattainable ideal, but it is our clear and 
permanent aim.” § 
2. The Quantum 
The problem of the quantum is not single but complex. In 
its inception the quantum theory was concerned with only one 
department of physics, but we have come to see that all branches 
of physics in which small-scale phenomena are investigated are 
concerned with quantum laws. The nature of radiation, the 
structure of the atom, the very existence of electric charges and 
the characteristics of the electro-magnetic field, all are dependent 
upon the quantum or upon quantum principles. 
The feature which forces itself upon our attention in con 
sidering the quantum is that of discontinuity referred to previously 
* See Rougier, Philosophy and the New Physics (Routledge), 
t Larmor, Aether and Matter, p. 334. 
+ Oliver Lodge, Nature, vol. 119, p. 423, 1927. § loc, cit.