f Uranus, or as conical refraction
from the undulatory theory of light.

You see I am not mincing matters, but avowing nakedly what many
scientific thinkers more or less distinctly believe. The formation of
a crystal, a plant, or an animal, is, in their eyes, a purely
mechanical problem, which differs from the problems of ordinary
mechanics, in the smallness of the masses, and the complexity of the
processes involved. Here you have one half of our dual truth; let us
now glance at the other half. Associated with this wonderful
mechanism of the animal body we have phenomena no less certain than
those of physics, but between which and the mechanism we discern no
necessary connection. A man, for example, can say 'I feel,' 'I
think,' 'I love;' but how does consciousness infuse itself into the
problem? The human brain is said to be the organ of thought and
feeling: when we are hurt, the brain feels it; when we ponder, or when
our passions or affections are excited, it is through the
instrumentality of the brain. Let us endeavour to be a little more
precise here. I hardly imagine there exists a profound scientific
thinker, who has reflected upon the subject, unwilling to admit the
extreme probability of the hypothesis, that for every fact of
consciousness, whether in the domain of sense, thought, or emotion, a
definite molecular condition, of motion or structure, is set up in the
brain; or who would be disposed even to deny that if the motion, or
structure, be induced by internal causes instead of external, the
effect on consciousness will be the same? Let any nerve, for example,
be thrown by morbid action into the precise state of motion which
would be communicated to it by the pulses of a heated body, surely
that nerve will declare itself hot--the mind will accept the
subjective intimation exactly as if it were objective. The retina may
be excited by purely mechanical means. A blow on the eye causes a
luminous flash, and the mere pressure of the finger on the external
ball produces a star of light, which Newton compared to the circles on
a peacock's tail. Disease makes people see visions and dream dreams;
but, in all such cases, could we examine the organs implicated, we
should, on philosophical grounds, expect to find them in that precise
molecular condition which the real objects, if present, would
superinduce.

The relation of physics to consciousness being thus invariable, it
follows that, given the state o