110 : - STONEY—-UNIVERSE OF REAL EXISTENCES. [April 3, 
CHAPTER 3. OF THE ABSOLUTE AND RELATIVE SIGNIFICATIONS 
OF TERMS. 
We may make use of Geometrical Optics for another purpose— 
to illustrate the variety of meanings which such words as existence, 
theory, hypothesis, actual, real, etc., may have. They are freely 
used in Geometrical Optics. They are there used in a relative 
sense, in subordination to the hypothesis that light consists of rays, 
which for the time being must be left unquestioned, and which we 
may call the master hypothesis, as it governs the use to be made of 
those terms. Thus we speak of ~ea/ rays in front of a mirror, and 
of wrtual rays behind it; we say that the true /Aeory is that the 
image on the retina is formed by rays reflected from the front of the 
mirror, but that it is legitimate to make the Ayforhesis that they 
emanate from a virtual image behind. When, however, we take 
the wider view that light is an electro-magnetic wxdulation, we 
recognize that what a moment ago we called real rays are not real, 
but a machinery substituted for what is real in the new sense that 
we have now to give to that word. For we are now using the term 
real in subordination to a much wider hypothesis, viz.: the great 
objective hypothesis that not only do our perceptions exist tempo- 
rarily, but also that each of those syntheta of perceptions which we 
call natural objects exists as a whole and persistently. And when 
we come in turn to recognize that this, in its turn, is not the true 
theory of existence, but only an eminently useful hypothesis— 
probably, indeed, the most useful hypothesis known to man—and 
when we find that we must advance a step behind it to reach the true 
theory of existence, then at last we reach the stage at which we may 
use the word vea/in its fullest abso/ute sense; if we succeed in acquir- 
ing a right to apply it to what is going on in the autic universe, the 
universe of real existences. Thus such terms as existence, theory, 
real, actual, etc., only attain their absolute, which is their fullest, 
meaning when applied to the events that go on in the Universe of 
Auta; and are to be understood in their objective, which is a rela- 
tive, sense when applied to what we regard as going on in that great 
objective hypotheton which we call nature; and in another still 
more removed relative sense when used in subordination to the nar- 
rower hypothesis which we have to entertain while investigating 
nature by the science of Geometrical Optics. When once this is 
clearly understood we are warned, and in some degree forearmed, 
