that "man does not
consist of two principles so essentially different from one another as
matter and spirit, which are always described as having _no one common
property_ by means of which they can affect or act upon each other." In
the "Systeme de la Nature," the same argument is often urged. 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. "The faculty
of thinking," says Dr. Priestley, "in general ripens and comes to
maturity with the body; it is also observed to decay with it,"--"If the
brain be affected, as by a blow on the head, by actual pressure within
the skull, by sleep, or by inflammation, the mental faculties are
universally affected in proportion. Likewise, as the mind is affected in
consequence