30 PHILOSOPHICAL SOCIETY OF WASHINGTON. 
In the first place, Prof. Tait infers that force can have no such 
reality as matter has, because it is to be reckoned positively and 
negatively—an action being opposed by reaction—while matter or 
mass is signless. This suggests two comments: (1), the author 
never questions the objective reality of space and time, of which 
realities it is an essential feature that to every direction or interval 
A-B, an equal direction or interval B—A, of opposite sign, corres- 
ponds; (2), the idea of a negative mass is not a self-contradictory 
one, and was once generally accepted. The element phlogiston was 
given up not because of any absurdity in ascribing levity to ma- 
terial substance, but because a form of matter with positive mass 
(oxygen), capable of explaining all the phenomena, had been actually 
separated and identified. 
Prof. Tait’s next criterion of objective reality is quantitative 
indestructibility, an attribute shared by time, space and matter, to 
which he adds energy. But the evidence of the indestructibility 
of energy is not of the same nature as that of the indestructibility 
of matter; for the latter in all its forms may be localized, and its 
density or elasticity measured; while the former, when stored up 
r “potential,” cannot be shown to possess any of the properties of 
energy kinetic, or any existence in space, or any objective character 
whatever. Prof. Tait admits this difficulty virtually, and awaits 
for its solution the discovery of some evidence “as yet unexplained, 
or rather unimagined.” All strains and other actions of a clock- 
weight on its supports are obviously precisely the same—or impalp- 
ably somewhat stronger—with the weight wound up an inch, as with 
it wound up a yard; and the existence of a greater “potential 
energy” in the latter case is to be found not in the clock, but in 
the mind, which requires this expression as a form in which to put 
its conviction that a certain greater amount of work can be obtained. 
Even though it be admitted that there are no other intelligible 
terms in which this conviction can be stated, it is clear that the 
indestructibility of energy is an ideal and subjective truth, and 
cannot, therefore, be relied on as evidence of a reality distinctively 
“objective.” 
A third point made by Prof. Tait against force is that its nume- 
rical expression is that of two ratios: “the space-rate of the trans- 
formation of energy” and “the time-rate of the generation of 
momentum.” These results are obtained by simple division, in an 
