INTRODUCTION. 
THROouGHOUT the ages there have been many philosophers 
great and small. The word “ Plato’’ may for convenience 
be taken to express and include the sum of them. Then, as 
scientific inquirers, we may well have for our device the well- 
known quotation: “ Plato is my friend, but truth is a greater 
friend.’’ We are searchers after truth, we revere our 
teachers; but not all our affectionate reverence or awe of 
great men, living or dead, can stay us in our quest. Truth 
alone is sovereign. 
The subject of religion is not dealt with' in this theory. 
We all agree that the ordinary- operations of nature are 
carried on under the rule of what we term “ natural law.”’ 
It is my endeavour to elucidate that law. 
The nomenclature adopted is that of the radium 
experimenters, because I wished to state the case as stated 
by them. There is not any magic in names; and it does not 
much matter what you call a thing so long as the term is 
clearly defined at the start, and you abide by that term and 
definition throughout the argument. Scientific terms are 
but words applied to temporary arrangements of matter 
caused by the speed, weight, and movements of material 
portions. This is to be remembered in order that we do 
not get lost in a forest of names, or allow scientific terms to 
obscure our vision of the reality and cause of things. 
Natural philosophy does not belong entirely to scientific 
experimentalists and mathematicians. When the first have 
made their delicate and difficult experiments, and the others 
have worked out their intricate problems, then, if either or 
both of them commence to construct a system of philosophy, 
