This is what I meant by saying that the great weakness of the hypothesis here criticized lies in its materialistic character. In contrast with this we shall presently see that the assertion of a future life which is not materially conditioned, though unsupported by any item of experience whatever, may nevertheless be an impregnable assertion. But first I would conclude the foregoing criticism by ruling out altogether the sense in which our authors use the expression "Unseen Universe." Scientific inference, however remote, is connected by such insensible gradations with ordinary perception, that one may well question the propriety of applying the term "unseen" to that which is presented to "the mind's eye" as inevitable matter of inference. It is true that we cannot see the ocean of ether in which visible matter floats; but there are many other 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.