ned, _brevi manu_, by the mere assumption of the
simplicity and uniform composition of everything in Nature; it can be
settled only by an appeal to the facts as they are known to exist. It is
the aim of science, undoubtedly, to reduce all compound substances to
the smallest possible number of constituent elements, and all complex
phenomena to the smallest possible number of general laws. But we feel
that, desirable as this simplification may be, we are not warranted in
identifying light with heat, or even electricity with magnetism, however
closely connected with each other, simply because there are certain
observed differences between them, which could not be explained, in the
present state of our knowledge, consistently with any such theory of
their absolute identity: and so, there are such manifest differences
between Mental and Material phenomena, that we cannot yield to the
temptation of ascribing them to one cause or origin, until it has been
satisfactorily proved that the same cause is sufficient to account for
appearances so diverse. It should be considered, too, in connection with
this pretence of greater simplicity, that even if we could succeed in
getting rid of the _dualism_ of Mind and Matter in the constitution of
man, we never can get rid of it with reference to the universe at
large, otherwise than by denying _the spirituality of God himself_: for
the grand, the indestructible, the eternal _dualism_ would still
remain,--the distinction between God and His works,--between the Creator
and the universe which He has called into being,--between the finite,
contingent, and transitory, and the infinite, necessary, and eternal.
And this is a distinction that cannot be obliterated, although it may be
obscured, by the speculations of Pantheism.

Another reason which has induced some to adopt, or at least to regard
with favor, the theory of Materialism, is--the difficulty of conceiving
of the union of two substances so incongruous as Mind and Matter are
supposed to be,--and still more the difficulty of explaining how they
could have any mutual action on each other. Dr. Priestley largely
insists on this, as well as on the former reason, as one of the main
inducements which led him to abandon the commonly-received doctrine.
"Many doubts occurred to me," he says, "on the subject of _the intimate
union of two substances so entirely heterogeneous_ as the soul and body
were represented to be." And he was led to conclude,