Mr. Wallace, in fact, appears to me to have mixed up two very distinct
propositions: the one, the indisputable truth that consciousness is
correlated with molecular changes in the organ of consciousness;
the other, that the nature of that correlation is known, or can be
conceived, which is quite another matter. Mr. Wallace, presumably,
believes in that correlation of phenomena which we call cause and
effect as firmly as I do. But if he has ever been able to form the
faintest notion how a cause gives rise to its effect, all I can say is
that I envy him. Take the simplest case imaginable--suppose a ball in
motion to impinge upon another ball at rest. I know very well, as a
matter of fact, that the ball in motion will communicate some of its
motion to the ball at rest, and that the motion of the two balls after
collision is precisely correlated with the masses of both balls and
the amount of motion of the first. But how does this come about? In
what manner can we conceive that the _vis viva_ of the first ball
passes into the second? I confess I can no more form any conception
of what happens in this case, than I can of what takes place when the
motion of particles of my nervous matter, caused by the impact of a
similar ball, gives rise to the state of consciousness I call pain. In
ultimate analysis everything is incomprehensible, and the whole object
of science is simply to reduce the fundamental incomprehensibilities
to the smallest possible number.

But to return to the Quarterly Reviewer. He admits that animals
have "mental images of sensible objects, combined in all degrees of
complexity, as governed by the laws of association." Presumably, by
this confused and imperfect statement the Reviewer means to admit
more than the words imply. For mental images of sensible objects,
even though "combined in all degrees of complexity," are, and can be,
nothing more than mental images of sensible objects. But judgments,
emotions, and volitions cannot by any possibility be included under
the head of "mental images of sensible objects."

If the greyhound had no better mental endowment than the Reviewer
allows him, he might have the "mental image" of the "sensible
object"--the hare--and that might be combined with the mental images
of other sensible objects, to any degree of complexity, but he would
have no power of judging it to be at a certain distance from him; no
power of perceiving its similarity to his memory of a hare; and