it is all a picture, prophesying what we might
see through a sufficiently powerful microscope; the important
philosophical question is the one raised by the other half of Locke's
natural philosophy, by optics and the general criticism of perception. How
far, if at all, may we trust the images in our minds to reveal the nature
of external things?

On this point the doctrine of Locke, through Descartes,[3] was also
derived from Democritus. It was that all the sensible qualities of things,
except position, shape, solidity, number and motion, were only ideas in
us, projected and falsely regarded as lodged in things. In the things,
these imputed or secondary qualities were simply powers, inherent in their
atomic constitution, and calculated to excite sensations of that character
in our bodies. This doctrine is readily established by Locke's plain
historical method, when applied to the study of rainbows, mirrors, effects
of perspective, dreams, jaundice, madness, and the will to believe: all of
which go to convince us that the ideas which we impulsively assume to be
qualities of objects are always, in their seat and origin, evolved in our
own heads.

These two parts of Locke's natural philosophy, however, are not in perfect
equilibrium. _All_ the feelings and ideas of an animal must be equally
conditioned by his organs and passions,[4] and he cannot be aware of what
goes on beyond him, except as it affects his own life.[5] How then could
Locke, or could Democritus, suppose that his ideas of space and atoms were
less human, less graphic, summary, and symbolic, than his sensations of
sound or colour? The language of science, no less than that of sense,
should have been recognised to be a human language; and the nature of
anything existent collateral with ourselves, be that collateral existence
material or mental, should have been confessed to be a subject for faith
and for hypothesis, never, by any possibility, for absolute or direct
intuition.

There is no occasion to take alarm at this doctrine as if it condemned us
to solitary confinement, and to ignorance of the world in which we live.
We see and know the world through our eyes and our intelligence, in visual
and in intellectual terms: how else should a world be seen or known which
is not the figment of a dream, but a collateral power, pressing and alien?
In the cognisance which an animal may take of his surroundings--and surely
all animals take such cognisance--the subje