Jan., 1889. 
spencer’s “ FIRST PRINCIPLES.” 
11 
taking them off again. But we apply the continuity postulate 
and assert that the weight existed while the masses were on 
the pan. But we go further : we assert constancy in quantity, 
which is something more than mere continuity of existence, 
and we have various methods of testing our assertion. We 
may allow the weights to fall, and time their fall through a 
given distance in supcessive trials. All experience tends to 
show that the time of fall is constant, and we conclude that 
the weight is constant. It may be argued that we use a clock 
for the time, and that the clock pendulum may possibly vary 
in weight, simultaneously and in like proportion with the 
balance weights. Very well, then ; let us use a watch, and we 
get still the same time of fall in our successive trials. Or 
let us use a different test, and put the weights on a Salter’s 
Spring balance, and they always stretch the spring equally. 
If it be argued that possibly the elasticity of the watch spring 
and of the balance spring varies in like proportion with the 
weight of the weights, then, I say, let us go to the ultimate 
court of appeal—my own sensations. If I have practised much 
with my pressure sense and my muscular sense, I may weigh 
the weights with my hand and be certain of their approximate 
constancy. If it be finally argued that my sensations may 
likewise vary in proportion, then I say that so long as the 
universe is drawn to a consistent scale, and so long as I am 
also on that scale, any contraction or expansion of the scale, 
being beyond my detection, is a matter of perfect indifference 
to me, and I need not construct my language so as to provide 
for its possibility. I am content to say that the weight of 
the salt and the water is constant. But here I think that 
another postulate has crept in, which in its most general form 
we may state thus—“ Like sensations imply like objective 
existences or like physical properties.” We use the particular 
case that equal sensations imply equal objective existences 
or equal physical properties. For whatever test of constancy 
of weight we employ depends ultimately on the equality of 
two sensations. And indeed this postulate is the basis of all 
the conclusions as to the outside world which we draw from 
physical measurements. 
Again, though we take weight as our test, the fact that the 
salt at the end resembles the salt with which we started in 
other respects than weight—in fact, that it gives us equal 
sensations—leads us to conclude that it is equal in quantity, 
i.e., that none of it has been destroyed. And on such 
experiments so interpreted we found the principle of the 
Indestructibility of Matter. 
(To be continued.) 
