20 PRESIDENT’S ADDRESS, 
we know no means of getting hold of it. The one thing we know 
metrical about it is the velocity with which it can transmit transverse 
waves. That is clear and definite, and thereby, to my judgment, it 
proves itself a physical agent; not indeed tangible or sensible, but 
yet concretely real. 
But it does elude our laboratory grasp. If we rapidly move 
matter through it, hoping to grip it and move it too, we fail: there 
is no mechanical connection. And even if we experiment on light 
we fail too. So long as transparent matter is moving relatively to 
us, light can be affected inside that matter; but when matter is 
relatively stationary to matter nothing observable takes place, how- 
ever fast things may be moving, so long as they move together. 
Hence arises the idea that motion with respect to Ether is mean- 
ingless: and the fact that only relative motion of pieces of matter 
with respect to each other has so far been observed is the foundation 
of the Principle of Relativity. It sounds simple enough as thus 
stated, but in its developments it is an ingenious and complicated 
doctrine, embodying surprising consequences, which have been worked 
out by Professor Einstein and his disciples with consummate ingenuity. 
What have I to urge against it? Well, in the first place, it is 
only in accordance with common sense that no effect of the first 
order can be observed without relative motion of matter. An Ether- 
stream through our laboratories is optically and electrically undetect- 
able, at least as regards first-order observation; this is clearly 
explained for general readers in my book ‘The Ether of Space,’ 
Chapter IV. (Also in Nature, vol. 46, p. 497.) But the Principle of 
Relativity says more than that; it says that no effect of any order of 
magnitude can ever be observed without the relative motion of matter. 
The truth underlying this doctrine is that absolute motion without 
reference to anything is unmeaning. But the narrowing down of 
‘anything’ to mean any piece of matter is illegitimate. The nearest 
approach to absolute motion that we can physically imagine is motion 
through or with respect to the Ether of Space. It is natural to 
assume that the Ether is on the whole stationary, and to use it as 
a standard of rest; in that sense motion with reference to it may 
be called absolute, but in no other sense. 
The Principle of Relativity claims that we can never ascertain such 
motion: in other words it practically or pragmatically denies the 
existence of the Ether. Every one of our scientifically observed 
motions, it says, are of the same nature as our popularly observed 
ones, viz., motion of pieces of matter relatively to each other; and 
that is all that we can ever know. Everything goes on—says the 
Principle of Relativity—as if the Ether did not exist. 
Now the facts are that no motion with reference to the ether 
