cannot in any way be made the objects of sense-perception; yet by
proper inferences from their behaviour we can single them out for
measurement, so that Sir William Thomson can tell us that if the
drop of water were magnified to the size of the earth, the
constituent atoms would be larger than peas, but not so large as
billiard-balls. If we do not see such atoms with our eyes, we
have one adequate reason in their tiny dimensions, though there
are further reasons than this. It would be hard to say why the
luminiferous ether should be relegated to the "unseen world" any
more than the material atom. Whatever we know as possessing
resistance and extension, whatever we can subject to mathematical
processes of measurement, we also conceive as existing in such
shape that, with appropriate eyes and under proper visual
conditions, we MIGHT see it, and we are not entitled to draw any
line of demarcation between such an object of inference and
others which may be made objects of sense-perception. To set
apart the ether as constituting an "unseen universe" is therefore
illegitimate and confusing. It introduces a distinction where
there is none, and obscures the fact that both invisible ether
and visible matter form but one grand universe in which the sum
of energy remains constant, though the order of its distribution
endlessly varies.

Very different would be the logical position of a theory which
should assume the existence of an "Unseen World" entirely
spiritual in constitution, and in which material conditions like
those of the visible world should have neither place nor meaning.
Such a world would not consist of ethers or gases or ghosts, but
of purely psychical relations akin to such as constitute thoughts
and feelings when our minds are least solicited by