There can be no reason for admitting the existence of "matter" as a substance, which does not apply also to the existence of "mind" as a distinct substance, if it shall be found that their properties are essentially different.

We know, and can know, nothing of _substance_ otherwise than by its properties or powers: we know nothing of "matter,"-- it would, in fact, be to us non-existent, but for its extension, solidity, and other properties; we know nothing of "mind,"-- it would equally be to us non-existent, but for its consciousness, its thoughts, feelings, and desires; and if it be right to ascribe the one set of properties to a substantive being, called "matter," it cannot be wrong to ascribe the other set of properties also to a substantive being, called "mind." If it could be shown, indeed, that the properties of the one substance might either be identified with, or accounted for, by those of the other; if animal feeling could be identified with or derived from, mere physical impulse; if intellectual thought could be reduced to material motion; if desire and aversion, hope and fear could be explained by the natural laws of attraction and repulsion, then we might blend the two substances into one, and speak of "mind" as a mere modification of "matter." But as long as the properties or powers by which alone any substance can be known are seen to be generically different, we cannot confound the substances themselves, or reduce them to one category, without violating the plainest rules of philosophical inquiry.
And yet to these rules Dr.Priestley refers, as if they warranted the conclusions at which he had arrived.

He desires his readers "to recur to the universally received rules of philosophizing, such as are laid down by Sir Isaac Newton at the beginning of his third book of "Principia." The first of these rules, as laid down by him, is that we are to _admit no more causes than are sufficient to explain appearances_; and the second is, that to _the same effect_ we must, as far as possible, assign _the same cause_." We cheerfully accept these canons of philosophical inquiry; and it is just because no one substance is sufficient, in our estimation, to account for _all_ the appearances, that we equally reject the "spiritualism" of Berkeley, who would resolve all phenomena into "mind," and the "materialism" of Priestley, who would resolve all phenomena into "matter." Matter and Mind may, indeed, be said to resemble each other in some respects,--in their being equally existent, equally created, and equally dependent; but their essential properties are generically different, for there is no identity, but a manifest and undeniable diversity, between thought, feeling, desire, volition, and conscience, and the various qualities or powers belonging to matter, such as extension, solidity, and _vis inertiae_, or even the powers of attraction and repulsion.

On the ground of this manifest difference between the properties by which alone any substance makes itself known, we hold ourselves warranted to affirm that the "mind" is immaterial, and to ascribe mental phenomena to a _distinct substantive being_, not less than the material phenomena of Nature.
Some ingenious thinkers, on both sides of the question, have not been fully satisfied with this method of stating the grounds of our opinion.
It has been said by our opponents, that if we found merely on the acknowledged difference between two sets of properties or phenomena, while we admit that the substance or substratum is in itself entirely unknown to us, or known only through the medium of the properties to which we refer,--then the dispute becomes a purely _verbal_ one, and can amount to nothing more than this, whether a _substance_ of whose essence we are entirely ignorant should be called by the name of "matter" or by the name of "spirit." But the dispute is not a purely _verbal_ one, even on the suppositions which have been stated.

For it is essential to a right "philosophy of nature," that every substance possessing peculiar properties should have a distinctive name.