r invisible things which yet we do not regard as part of
the "unseen world." I do not see the air which I am now breathing within
the four walls of my study, yet its existence is sufficiently a matter
of sense-perception as it fills my lungs and fans my cheek. The atoms
which compose a drop of water are not only invisible, but 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 sense-perceptions. In thus marking off the "Unseen
World" from the objective universe of which we have knowledge, our
line of demarcation would at least be drawn in the right place. The
distinction between psychical and material phenomena is a distinction of
a different order from all other distinctions known to philosophy, and
it immeasurably transce