46 PHILOSOPHICAL SOCIETY OF WASHINGTON, 
essence is manifested to us. We know them provisionally as forces, 
in the Newtonian nomenclature. Had gaseous matter neither 
weight nor mass, we could not know of its existence. But these 
attributes are so constant in matter that we estimate its quantity 
in terms of them and have no other exact terms. Weight is the 
statical measure; mass the dynamical measure. And since weight 
and mass correspond for all substances, under all transformations, 
we judge that the correspondence identifies them alike with the 
essence. They cannot be the mere result of organization. They 
must belong to the ultimate atom. 
At this point it would seem proper to attend to a question of defi- 
nition. Definitions are essential to clearness, on the one hand, and 
a source of entanglement on the other, if we fall into the scholastic 
error of regarding a mere word as the coextensive symbol of an 
idea. Words are evolved during the imperfection of ideas, and 
language is still a most imperfect medium of expression. Hence, 
logic is not a science in the sense that mathematics is. I have used 
the term force. This is a word of much ambiguity of meaning. 
We may use it as a convenient mathematical expression fur a mere 
rate of change of momentum, or we may go farther and define it, 
as that which changes a body’s state of rest or of uniform motion 
in a straight line; either of which uses restricts it to only a portion 
of phenomena, and ignores the whole science of statics, dealing with 
forces in equilibrium and the phenomena of balanced stress. If we 
give it a more general signification, as that which changes or tends 
to change, or conserve, the state of motion of particles, or systems 
of such, either in quantity or direction, we embrace statics as well 
as kinematics, and get a measurably philosophical definition, if we 
bear in mind the proviso that we do not thereby postulate force as 
an entity apart from substance. 
And since the compound variable space and time condition which 
we call motion (of which rest is but a phase) is the sensible result- 
ant of the interaction of such discrete substance by constant rear- 
rangement where readjustment is free, or the potential resultant 
where confined, we may admit that the observed tension and per- 
sistence, of whatever form, is that which effects the phenomenon 
(though masked by infinite variety and composition), and always 
across the discontinuity: not as separate entities, but as modes of 
manifestation of the interacting and pervasive substance itself and 
