It is boldly and repeatedly affirmed that "an immaterial cause cannot produce motion;" and this is applied equally to the soul and to God.

"How can we form an idea of a substance destitute of extension, and yet acting on our senses, that is, on material organs which are extended?
How can a being without extension be capable of motion, and of putting matter into motion ?"--"It is as impossible that spirit or thought should produce matter, as that matter should produce spirit or thought."[154] Now, it is not denied by any,--it is admitted on all hands,--that the union between the soul and the body is a great mystery, and that we are not able, in the present state of our knowledge, to explain either the action of matter on mind, or the action of mind on matter.

The mode of the union between them, and the nature of the influence which they mutually exercise, are to us inscrutable: but _the facts_ of our most familiar experience are not the less certain, because they depend on causes to us unknown, or stand connected with mysteries which we cannot solve.

Besides, the theory of _unisubstancisme_ itself, were it adopted, would still leave many facts unexplained, and the inmost nature of man would continue to be as inscrutable as before.

There is nothing inconceivable, impossible, or self-contradictory in the supposition of a non-material or spiritual substance; nor is there any reason _a priori_ to conclude that such a substance could not be united to a material frame, although the nature of their union, and the mode of their reciprocal action, might be to us inexplicable.
There is still another reason which is urged by some, derived from _the dependence of the mind on the body_, and its liability to be affected, beneficially or injuriously, by mere physical influences.