38 PRESIDENT S ADDRESS—SECTION A. 
significant or valid for the purpose of proposed ultimate ex- 
planations ; it ceases to be intelligible. To use conception as a 
means of deducing the very property, which is first assumed, 
nay, even explicitly contained in its definition, is either a petitio 
principri, or a vicious circle, of the most glaring character. 
This is a question to which we shall later return. 
40. The Law of Parsimony the Basis of Explanation.—The 
effort to explain natural phenomena by means of the smallest 
possible number of hypotheses—.e., assumptions not susceptible 
of direct proof by experiment—is, of course, the necessary aim 
of science. The justification of this is what has been called the 
law of parsimony; in other words, we must be niggardly with 
our hypotheses, and may not create two where one is sufficient. 
Failing to adhere to this as a principle, there could be no ideal 
fmality as to modes of explanation; explanation would have no 
organic unity. Thus the value of Newton’s hypothesis of umi- 
versal gravitation was that it afforded a purely mechanical 
explanation of the complex motions, at least in the whole solar 
system (€); and of so great a range of facts in the realm of 
natural philosophy. In this way it has not only enormously 
enhanced our powers of prediction as to the course of natural 
phzenomena, it has also subsumed them under a great general 
conception. This last is the highest service. 
41. Origin of the Conceptions of Mass and Energy.—The 
ideas of mass and energy are, of course, formulated out of the 
contents of sense experience. Through that of muscular resist- 
ance we acquire the notions of spatial extension, of inertia, and 
of force, from which that of energy can be deduced by abstract 
considerations ; that is to say, we learn through the muscular 
sense that masses have extension in space, and that effort is 
necessary to npart velocity to them, or, to put it more gene- 
rally, to change their velocities. Terrestrially, also, we learn the 
eravitative action of tle earth’s mass in a most impressive way, 
and by delicate experiment can measure the attraction of 
ordinary masses. We thus connect the idea of force with that 
property of mass which causes it to act on other distant masses 
so as to produce motion in them, or manifest itself as energy. 
We have also, through direct perception, the idea of action at a 
distance, which, I submit, we may put out of sight by effort of 
imagination, but never can get rid of in reality (/). 
Actio in Distans, and Impact.—for some reason, which 
is by no means obvious, it has often been tacitly assumed that 
in the mechanical explanation of phenomena the introduction 
(e) There are some difiiculties, however, about the sufficiency of the Newtonian law in 
this connection, which are occupying the attention of mathematical astronomers at the 
present time, and it is questionable whether the law of the inverse square of the distance 
is exact. The law, as stated by Newton, may be only approximate. 
(7) One is reminded of the classical example of actio in distans: A boy gazing at an 
apple. Result: Motion generated in the boy ; later in the apple. 
