e of our
idea of causality (the mind); not from that into which this idea has
been read by the mind. Hence, it is so far less difficult to imagine
that mental changes are the cause of bodily changes than _vice versa_;
for upon this hypothesis we are starting at least from the substance of
immediate knowledge, and not from the reflection of that knowledge in
what we call the external world.

On the other hand, the theory of Spiritualism labours under certain
speculative difficulties which appear to me overwhelming. The most
formidable of these difficulties arises from the inevitable collision of
the theory with the scientific doctrine of the conservation of energy.
Whether or not we adopt the view that all causation of a physical kind
is ultimately an expression of the fact that matter and energy are
indestructible[3], it is equally certain that this indestructibility is
a necessary condition to the occurrence of causation as natural.
Therefore, if the mind of man is capable of breaking in as an
independent cause upon the otherwise uniform system of natural
causation, the only way in which it could do so would be by either
destroying or creating certain _quanta_ of either matter or energy or
both. But to suppose the mind capable of doing any of these things would
be to suppose that the mind is a cause in some other sense than a
physical or a natural cause; it would be to suppose that the mind is a
super-natural cause, or, more plainly, that all mental activity, so far
as it is an efficient cause of bodily movement, is of the nature of a
miracle.

This conclusion, which appears to me unavoidably implicated in the
spiritualistic hypothesis, is not merely improbable _per se_, but admits
of being shown virtually impossible if we proceed to consider the
consequences to which it necessarily leads. A sportsman, for example,
pulls the trigger of a gun, thereby initiating a long train of physical
causes, which we may take up at the point where the powder is
discharged, the shot propelled, and the bird dropped. Here the man's
volition is supposed to have broken in upon the otherwise continuous
stream of physical causes--first by modifying the molecular movements of
his brain, so as to produce the particular co-ordination of
neuro-muscular movement required to take accurate aim and to fire at the
right moment; next by converting a quantity of gunpowder into gas,
propelling a quantity of lead through the air; and finally, by killi