In discussing the former hypothesis, Dr. Priestley denies not only the existence of spirit as having no relation to extension or space, but also the common definition of matter, as a substance possessing only the inert properties of extension, and solidity or impenetrability. The latter he defines in conformity with the more accurate observations of later physics, a substance possessing the property of extension and the active powers of attraction and repulsion. With Boscovich and Mr. Michell, he admits of the penetrability of matter, and replies to the objections that may be drawn from this view of the subject. It must be acknowledged that highly curious as this preliminary disquisition is, it is not only unnecessary to the main argument, but leaves the definition of matter open to the question whether there be any substratum or subject in which the essential properties or powers of attracting and repelling inhere. That these powers really belong to matter, whatever else matter may be, is evident from the reflection of light, previous to contact with the reflecting substance and its inflection afterward from the electric spark, visible along a suspended chain, from the phenomena of the metallic pyrometers, from the rain drop on a cabbage leaf, &c. And that matter is permeable, at least to light, is sufficiently evident from every case of tranparency. Still however it cannot consist of properties alone; a property must be the property of something. But the proper and direct train of argument in favour of materialism is, that every phenomenon from which the notion of a soul is deduced, is resolveable into some affection of the brain, perceived. That all thought, reflection, choice, judgment, memory, the passions and affections, &c. consist only of ideas or sensations,(i. e. motions within that organ) perceived at the time. Though, judgment, memory, being words, denoting different kinds of internal perceptions, relating only to, and consisting of, ideas and sensations.[67] That sensations and ideas themselves, arise only in consequence of the impressions of external objects on our senses, which impressions are liable to be recalled afterward by the recurrence of others with which they were originally associated, agreeably to the necessary and inevitable law of the animal system. That this is evident in as much as there can be no ideas peculiar to any of the senses where there is a want of the necessary bodily organ, as of hearing, sight, &c. inasmuch as all these ideas commence with the body, grow with its growth, and decrease with its decline. That they can be suspended, altered, destroyed, by artificial means, by accident, by disease. That all these properties of mind, viz. thought, judgment, memory, passions, and affections, are as evident in brutes as in men; and though the degree be different, it is always accompanied with a proportionate difference of organization. That perception is clearly the result of organization,