334 27m Philippine Journal of Science 1921 
of these conceptions are very valuable, and certain relations 
between them, such as the law of conservation of energy 
(especially if we are particular always to define energy in such 
a manner that this law will not be violated) , are fundamentally 
necessary to science. The fact, however, that one could in a 
single day define as many as a thousand such conceptions 
hitherto unused, and could spend a lifetime in developing new 
equations expressing the relations between these quantities, 
even if they all applied only to a single rifle bullet considered as 
a rigid body, should prevent us from setting up any such con- 
ception or any such relation as a “graven image to bow down 
and worship it.” 
Presumably none of our ideas are absolutely correct, but com- 
mon-sense reasoning tells us that some, like Euclid’s and Max- 
well’s laws, are intimately connected with the real structure 
of the universe; while others, like the statement that the en- 
tropy of a system always tends to a maximum, or e = hv, bear 
distinct trademarks of human manufacture. I think Minkowski 
has conclusively demonstrated the value of considering that our 
concept of space is only a partial view of a more fundamental 
four-dimensional construction of the universe. I may add that it 
does not follow that time is not an entity independent of this 
four-dimensional construction. For we can measure time in 
absolute units (see following paper) wherever we may be sit- 
uated in this four-dimensional construction, from which fact I 
can form no other conclusion but that time, if correctly defined, 
is independent of the four-dimensional construction. However 
this may be, the four-dimensional construction has not shaken 
our faith that Maxwell’s and Euclid’s laws represent close ap- 
proximations to the fundamental construction of the universe, 
but rather has increased our reason for belief that these laws, 
in the extended four-dimensional forms (which, for the purposes 
of the present paper, are equivalent to the three-dimensional 
forms with the Lorentz transformation), are very fundamental. 
Therefore, it seems to me the height of folly so to concentrate 
our attention on one of the laws of the second class mentioned 
that we fail to apply to our problems the known laws which are 
of a much more fundamental nature. 
Common-sense logic (which of course is always subject to 
correction) tells us that a light wave is the motion* of some- 
thing caused by the motion of something else. If we find a 
peculiar relation between light waves, it is natural to refer it 
to the properties of one of these things. It has been for some 
