Not content with the generalization, alike safe and legitimate, which ranks both mind and matter under the generic head of _substance_, they have sought to reduce them to the same category, and to give to matter a monopoly of the universe, at least of created being.
In support of their views, they remind us of the fundamental principle of philosophy as laid down by Sir Isaac Newton, that "we are to admit no more causes of things than are sufficient to explain appearances."[153] The principle is a sound one; and the only question is, whether matter alone is sufficient to account for mental phenomena?
On _this_ question the two parties are at irreconcilable variance; and the controversy cannot be determined, _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, 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.