394 
Another illustration of the illegitimate 
use of our hypothesis, as it appears to me, 
is in the attempt to find in the ether a fixed 
datum for the measurement of material 
velocities and accelerations, a something in 
which we can draw our coordinate axes so 
that they will never turn or bend. But 
this is as if, discontented with the move- 
ment of the earth’s pole, we should seek to 
find our zero lines of latitude and longitude 
in the Atlantic Ocean. Leaving out of 
sight the possibility of ethereal currents 
which we cannot detect, and the motions 
due to every ray of light which traverses 
space, we could only fix positions and direc- 
tions in the ether by buoying them with 
matter. We know nothing of the ether, 
except by its effects on matter, and, after 
all, it would be the material buoys which 
would fix the positions and not the ether 
in which they float. 
The discussion of the physical method, 
with its descriptive laws and explanations, 
and its hypothetical extension of descrip- 
tion, leads us on to the consideration of the 
limitation of its range. The method was 
developed in the study of matter which we 
describe as non-living, and with non-living 
measure more real than another, more in accordance 
with the truth of things, may be further illustrated 
by the common idea that mass-acceleration is the only 
way to measure a force. We stand apart from our 
mechanical system and watch the motions and the ac- 
celerations of the various parts, and we find that 
mass-accelerations have a certain significance in our 
system. If we keep ourselves outside the system and 
only use our sense of sight, then mass-acceleration is 
the only way of describing that behavior of one body 
in the presence of others which we term force on it. 
But if we go about in the system and pull and push 
bodies, we find that there is another conception of 
force, in which another sense than sight is concerned 
—another mode of measurement much more ancient 
and still far more extensively used—the measurement 
by weight supported. Each method has its own 
range ; each is fundamental in that range. It is one 
of the great practical problems in physics to make the 
pendulum give us the exact ratio of the units in the 
two systems. 
SCIENCE. 
[N. 8. Vou. X. No. 247. 
matter the method has sufficed for the par- 
ticular purposes of the physicist. Of course 
only a little corner of the universe has been 
explored, but in the study of non-living 
matter we have come to no impassable 
gulfs, no chasms across which we cannot 
throw bridges of hypothesis. Does the 
method equally suffice when it is applied 
to living matter? Can we give a purely 
physical account of such matter, likening its 
motions and changes to other motions and 
changes already observed, and so explaining 
them? Can we group them in laws which 
will enable us to predict future conditions 
and positions ?—The ancient question never 
answered, but never ceasing to press for 
an answer. 
Having faith in our descriptive method, 
let us use it to describe our real attitude on 
the question. Dowe, or do we not, as a 
matter of fact, make any attempt to apply 
the physical method to describe and explain 
those motions of matter which on the psy- 
chical view we term voluntary ? 
Any commonplace example, and the more 
commonplace the more it is to the point, 
will at once teli us our practice, whatever 
may be our theory. For instance, a steamer 
is going across the Channel. We can give 
a fairly good physical account of the motion 
of the steamer. We can describe how the 
energy stored in the coal passes out through 
the boiler into the machinery, and how it 
is ultimately absorbed by the sea. And the 
machinery once started, we can give an ac- 
count of the actions and reactions between 
its various parts and the water, and if only 
the crew will not interfere, we can predict 
with some approach to correctness how the 
vessel will run. All these processes can be 
likened to processes already studied—per- 
haps on another scale—in our laboratories, 
and from the similarities prediction is pos- 
sible. But now think of a passenger on 
board who has received an invitation to 
take the journey. Itis simply a matter of 
