f Existence in general,
nevertheless has recourse to inventing a wholly gratuitous hypothesis to
explain one mode of existence in particular. If it is said that the latter
hypothesis has the merit of causing the mystery of material existence and
the mystery of mental existence to be united in a thinkable manner--viz.,
in a self-existing Mind,--I reply, It is not so; for in whatever degree it
is unthinkable that Matter should be the cause of Mind, in that precise
degree must it be unthinkable that Mind was ever the cause of Matter, the
correlatives being in each case the same, and experience affording no
evidence of causality in either.

Sec. 14. The two hypotheses, therefore, are of exactly equivalent value, save
that while the one has a certain basis of fact to rest upon,[7] the other
is wholly arbitrary. But it may still be retorted, 'Is not that which is
_most_ conceivable _most likely_ to be true? and if it is more conceivable
that my intelligence is caused by another Intelligence than that it is
caused by Non-intelligence, may I not regard the more conceivable
hypothesis as also the more probable one? It is somewhat difficult to say
how far this argument is, in this case, valid; only I think it is quite
evident that its validity is open to grave dispute. For nothing can be more
evident to a philosophical thinker than that the substance of Mind must--so
far at least as we can at present see--_necessarily_ be unknowable; so that
if Matter (and Force) be this substance, we should antecedently expect to
find that the actual causal connection should, in this particular case, be
more inconceivable than some imaginary one: it would be more natural for
the mind to infer that something conceivably more akin to itself should be
its cause, than that this cause should be the entity which really gives
rise to the unthinkable connection. But even waiving this reflection, and
granting that the above argument is _valid_, it is still to an indefinite
degree _valueless_, seeing that we are unable to tell _how much it is more
likely_ that the more conceivable should here be true than that the less
conceivable should be so.

Sec. 15. Returning then to Locke's comparison between the certainty of this
argument and that which proves the sum of the angles of a triangle to be
equal to two right-angles, I should say that there is a _virtual_, though
not a _formal_, fallacy in his presentation. For mathematical science being
confessedly but