of EdinhimjJi, Session 1883-84 5G9 
conception eitlier of rest or of rectilinear motion throngli unmarked 
space. 
We have besides no preliminary knowledge of any principle of 
chronometry, and for this additional reason we are under an essential 
preliminary difficulty as to attaching any clear meaning to the words 
uniform rectilinear motion as commonly employed, the uniformity 
being that of equality of spaces passed over in equal times. 
If two balls are altering their distance apart, v/e cannot suppose 
that they are both at rest. One, at least, must be in motion. 
Men have very good means of knowing in some cases, and of 
imagining in other cases, th,e distance between the points of space 
simultaneously occupied by the centres of two balls ; if, at least, we 
be content to waive the difficulty as to imperfection of our means of 
ascertaining or specifying, or clearly idealising, simultaneity at 
distant places. For this we do commonly use signals by sound, by 
light, by electricity, by connecting wires or bars, and by various 
other means. The time required in the transmission of the signal 
involves an imperfection in human powers of ascertaining simul- 
taneity of occurrences in distant places. It seems, however, pro- 
bably not to involve any difficulty of idealising or imagining the 
existence of simultaneity. Probably it may not be felt to involve 
any difficulty comparable to that of attempting to form a distinct 
notion of identity of place at successive times in unmarked space. 
There is, in the nature of things, a real distinction, cognisable by 
men, between absolute rotation (or absolute clinural motion) and 
absolute freedom from rotation (or absolute clinural rest).* 
The only motion of a point that men can know of or can deal 
with is motion relative to one, two, three, or more other points. 
Three points marked or indicated on one, two, or three bodies, the 
centres, for instance, of three balls, whether preserving their dis- 
tances apart, unchanging or not, are sufficient for enabling us to 
construct or to imagine a reference frame of any changeless configu- 
ration desired — three rectangular co-ordinate axes, for instance, or 
* The word clinural is to be understood as introduced for conveying pre- 
cisely one out of the various conflicting meanings of the word directional. All 
straight lines which are mutually parallel are, in this amended mode of nomen- 
clature, said to be in one same clinurc. In connection with this, it may l)e 
convenient here to mention that all parallel planes are, in like manner, said 
to be in one same Closure. 
