572 
Proceedioigs of the Royal Society 
ISTow, reverting to the essential difficulty already mentioned as to 
our forming a distinct conception either of rest or of uniform recti- 
linear motion, we may go forward to some further considerations and 
scrutinies as to what inen can imagine or can, really know through 
observation and experience respecting motions of bodies in the 
universe of space. 
We may have a firm persuasion, even without perfect understand- 
ing, that in the nature of things there niu,st be a reality correspond- 
ing to our glimmering idea of motion of a body along a straight 
course with changeless velocity, and that there must be an essential 
clistinction between such motion and motion along a curved course 
or motion with varying velocity. We cannot, however, specify 
such motions relatively to unmarked space and unmeasured passage 
of time. We cannot specify them as to any condition of absolute 
rest. We can only specify them as to part of their characters, or 
conditions, or distinctions. We can do so only in so far as quali- 
ti,es or distinctions of motions of one or more bodies can be ascer- 
tained through knowable relations between these motions and the 
motions of one or more other bodies. Briefly, we can deal only 
with relative motions or relative rest ; not with absolute motions nor 
absolute rest. 
Sir Isaac Newton sets forth, under the designation of the First 
Law or Motion, the statement that — Every body continues in its 
state of resting or of moving uniformly in a strciight course, except in 
so much as, hy applied forces, it is compelled to. change that state. 
A most important truth in the nature of things, perceived with 
more or less clearness, is at the root of this enunciation, but the 
\yords, whether taken by themselves or in connection with Newton’s 
prefatory and accompanying definitions and illustrations, are inade- 
quate to give expression to that great natural truth. In attempting 
to draw from the statement a perfectly intelligible conception, we 
find ourselves confronted with the preliminary difficulty or im- 
possibility as to forming any perfectly distinct notion of a meaning- 
in respect to a single body, for the phrase ‘‘ stgte of resting or of 
ryoving uniformly, in a straight course.” Newton’s previous assertion 
tfuit there exists absolute space, which, in its own nature, without 
reference to anything else, always remains alike ayd immovable, does 
not clear away the difficulty. It does not do so, because it involves 
