SUBTLE IS THE LORD 127believe they have this intuition are the dupes of an illusion' (the italics are Poin-
care's). He further remarks, 'It is difficult to separate the qualitative problems of
simultaneity from the quantitative problem of the measurement of time; either one
uses a chronometer, or one takes into account a transmission velocity such as the
one of light, since one cannot measure such a velocity without measuring a time.'
After discussing the inadequacies of earlier definitions of simultaneity, Poincare
concludes, 'The simultaneity of two events or the order of their succession, as well
as the equality of two time intervals, must be defined in such a way that the state-
ments of the natural laws be as simple as possible. In other words, all rules and
definitions are but the result of an unconscious opportunism.' These lines read
like the general program for what would be given concrete shape seven years later.
Other comments in this paper indicate that Poincare wrote this article in response
to several other recent publications on the often-debated question of the measure-
ment of time intervals. The new element which Poincare injected into these dis-
cussions was his questioning of the objective meaning of simultaneity.
In 1898 Poincare did not mention any of the problems in electrodynamics. He
did so on two subsequent occasions, in 1900 and in 1904. The style is again pro-
grammatic. In these works, the aether questions are central. 'Does the aether
really exist?' he asked in his opening address to the Paris Congress of 1900 [P7].*
'One knows where our belief in the aether stems from. When light is on its way
to us from a far star ... it is no longer on the star and not yet on the earth. It is
necessary that it is somewhere, sustained, so to say, by some material support.' He
remarked that in the Fizeau experiment 'one believes one can touch the aether
with one's fingers.' Turning to theoretical ideas, he noted that the Lorentz theory
'is the most satisfactory one we have.'** However, he considered it a drawback
that the independence of optical phenomena from the motion of the earth should
have separate explanations in first and in second order. 'One must find one and
the same explanation for one and for the other, and everything leads us to antic-
ipate that this explanation will be valid for higher-order terms as well and that
the cancellation of the [velocity-dependent] terms will be rigorous and absolute.'
His reference to cancellations would seem to indicate that he was thinking about
a conspiracy of dynamic effects.
In 1904 he returned to the same topics, once again in a programmatic way, in
his address to the International Congress of Arts and Science at St. Louis [P9].f
'What is the aether, how are its molecules arrayed, do they attract or repel each
other?' He expressed his unease with the idea of an absolute velocity: 'If we suc-
ceed in measuring something we will always have the freedom to say that it is not
*This address is available in English as Chapters 9 and 10 in Science and Hypothesis [P8j.
"'During the period 1895 to 1900, Poincare considered it a flaw of the theory that it did not satisfy
momentum conservation in the Newtonian sense, that is, conservation of momentum for matter only.
He withdrew this objection soon afterward.
-(•This address is available in English as Chapters 7 to 9 in The Value of Science [P6].