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This hypothesis, however, is in no way necessary; and in a
few other rare cases in which similar hypotheses have had to be set
up, experiment has always in the long run enabled us to discover some
phenomenon which had escaped the first observers and which corresponds
exactly to the variation of energy first made evident.
One difficulty, however, arises from the fact that the principle ought
only to be applied to an isolated system. Whether we imagine actions
at a distance or believe in intermediate media, we must always
recognise that there exist no bodies in the world incapable of acting
on each other, and we can never affirm that some modification in the
energy of a given place may not have its echo in some unknown spot
afar off. This difficulty may sometimes render the value of the
principle rather illusory.
Similarly, it behoves us not to receive without a certain distrust the
extension by certain philosophers to the whole Universe, of a property
demonstrated for those restricted systems which observation can alone
reach.


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