Chapter XXX: Part II: , chap. iii., of his Sixth Book, having first said that

matter can not have the power of moving itself, he proceeds to
argue that neither can mind have the power of moving it. “Quand on
examine l’idée que l’on a de tous les esprits finis, on ne voit
point de liaison nécessaire entre leur volonté et le mouvement de
quelque corps que ce soit, on voit au contraire qu’il n’y en a
point, et qu’il n’y en peut avoir” (there is nothing in the idea
of finite mind which can account for its causing the motion of a
body); “on doit aussi conclure, si on vent raisonner selon ses
lumières, qu’il n’y a aucun esprit créé qui puisse remuer quelque
corps que ce soit comme cause véritable on principale, de même que
l’on a dit qu’aucun corps ne se pouvait remuer soi-même:” thus the
idea of Mind is according to him as incompatible as the idea of
Matter with the exercise of active force. But when, he continues,
we consider not a created but a Divine Mind, the case is altered;
for the idea of a Divine Mind includes omnipotence; and the idea
of omnipotence does contain the idea of being able to move bodies.
Thus it is the nature of omnipotence which renders the motion of
bodies even by the Divine Mind credible or conceivable, while, so
far as depended on the mere nature of mind, it would have been
inconceivable and incredible. If Malebranche had not believed in
an omnipotent Being, he would have held all action of mind on body
to be a demonstrated impossibility.(131)

A doctrine more precisely the reverse of the Volitional theory of
causation can not well be imagined. The Volitional theory is, that
we know by intuition or by direct experience the action of our own
mental volitions on matter; that we may hence infer all other
action upon matter to be that of volition, and might thus know,
without any other evidence, that matter is under the government of
a Divine Mind. Leibnitz and the Cartesians, on the contrary,
maintain that our volitions do not and can not act upon matter,
and that it is only the existence of an all-governing Being, and
that Being omnipotent, which can account for the sequence between
our volitions and our bodily actions. When we consider that each
of these two theories, which, as theories of causation, stand at
the opposite extremes of possible divergence from one another,
invokes not only as its evidence, but as its sole evidence, the
absolute inconceivability of any theory but itself, we are enabled
to measure the worth of this kind of evidence: and when we find
the Volitional theory entirely built upon the assertion that by
our mental constitution we are compelled to recognize our
volitions as efficient causes, and then find other thinkers
maintaining that we know that they are not and can not be such
causes, and can not conceive them to be so, I think we have a
right to say that this supposed law of our mental constitution
does not exist.

Dr. Tulloch (pp. 45–47) thinks it a sufficient answer to this,
that Leibnitz and the Cartesians were Theists, and believed the
will of God to be an efficient cause. Doubtless they did, and the
Cartesians even believed (though Leibnitz did not) that it is the
only such cause. Dr. Tulloch mistakes the nature of the question.
I was not writing on Theism, as Dr. Tulloch is, but against a
particular theory of causation, which, if it be unfounded, can
give no effective support to Theism or to any thing else. I found
it asserted that volition is the only efficient cause, on the
ground that no other efficient cause is conceivable. To this
assertion I oppose the instances of Leibnitz and of the
Cartesians, who affirmed with equal positiveness that volition as
an efficient cause is itself not conceivable, and that
omnipotence, which renders all things conceivable, can alone take
away the impossibility. This I thought, and think, a conclusive
answer to the argument on which this theory of causation avowedly
depends. But I certainly did not imagine that Theism was bound up
with that theory; nor expected to be charged with denying Leibnitz
and the Cartesians to be Theists because I denied that they held
the theory.