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A third hypothesis assumes that gravitation results from 
unequal pressure of the ether on the inner and outer side of 
each pair of masses or atoms. This is the view modestly 
proposed in Newton's 21st query. But his mind could not 
have found rest in it, since later on he inclines to a different 
and really opposite view. The one thing of which he seems to 
be sure is the exact converse of modern materialism. The main 
business, he says, of Natural Philosophy is to argue from 
phenomena, and deduce causes from effects, “till we come to 
the First Cause, which is certainly not mechanical." 
But this attempt to explain gravity, either by vibrations of 
ether, or differences of ethereal pressure, in spite of the high 
names which have inclined to it or adopted it, seems ‘to 
me open to a decisive and fatal objection. The action of the 
ether is assumed to depend on variations in its density. It 
would press equally on all sides, and be inactive, if its density 
were uniform. Now in ether, which was a plenum, no 
differences of density could exist. Space could not be more 
than perfectly full. And in elastic ether, not a plenum, the 
chief effect of the elasticity must be to equalize the density, 
and reduce the differences to nothing. While this change 
was in progress, the result must be to increase the mutual 
distance of all the matter floating in the denser portions, and 
to bring nearer to each other those which were placed in the 
rarer portions only. Thus, instead of universal attraction, 
the necessary result would be attraction oi' nearer approach in 
one half of space, and repulsion or further separation in the 
other half, and by a law or rule wholly differing in both from 
the inverse square of the distance. And when once an equal 
density of the ether was attained, or nearly attained, all further 
action must cease. The final result could be nothing else 
than stagnation, silence, and death. 
But if gravitation be an ultimate law, and cannot be 
resolved into a secondary result of impact or pressure, as I 
fully believe, a further doubt remains. Is attraction its true 
and proper name ? When A and B are in presence, and B 
draws nearer to A, does A pull B towards it ? Then the law 
is rightly called one of universal attraction. Or does B seek 
A and draw nearer to it by an inward instinct or impulse ? 
I hen the proper name of the law will be universal appetency. 
This last, though not the usual, I hold to be the more natural 
and reasonable view. It places the activity where the change 
occurs, not in every other place beside. It also brings the 
law into harmony with the higher forms of desire and appe- 
tite in all living creatures. Instead of a type of selfishness, 
