Akin to the inadvertence which, as I presume to think, has led Professor Huxley thus to misconceive _secondary_ qualities, is an inattention to the differences between our ideas, or mental pictures, and the originals whereof those pictures are copies, which seems to me seriously to vitiate his reasoning with regard to _primary_ qualities. With admirable perspicuity he shows[34] how it is that our notions of primary qualities are formed; how the mind, by _localising_ on distinct points of the sensory surface of the body its various, tactile sensations, obtains the idea of extension, or space in two dimensions, of figure, number, and motion: how the power, combined with consciousness of the power, of moving the hand in all directions over any substance it is in contact with, adds the idea of geometrical solidity, or of space in three dimensions: how the ideas thus formed with the aid of the sense of touch are confirmed by, and blended with, others derived from visual sensations and muscular movements of the eye: and, finally, how the idea of mechanical solidity, or impenetrability, arises from experience of resistance to our muscular exertions. All these details, however, interesting as they are, are nevertheless quite out of place. What we are at present concerned with is the nature of the things themselves, not the nature of our knowledge of them. No question that this latter is purely mental. If figure, motion, and solidity were really, as Professor Huxley says, each of them nothing but a perception of the relation of two or more sensations to one another, no question but that, since the mind is the sole seat of perception, they could exist nowhere else. But if all these suppositions be incorrect, if, as we have seen, there be in matter and apart from mind, potentialities of producing sensations, it follows that, in matter, and outside of mind, there must be relations between different potentialities, and there must, moreover, be limits to, and there may be changes in, those relations. Wherefore, since there is in matter a potentiality of imparting to the mind those sensations whence it derives its ideas of place and distance, and since figure is but a 'limitation of distance,' and motion but a 'change of place,' it necessarily follows that there is in matter a potentiality of conveying to the mind those sensations whence it derives its ideas of figure and motion. And a similar remark applies equally to solidity, and to every other so-called quality of matter. All of them are substantive potentialities of producing in the mind those sensations whence our ideas of themselves (the qualities) are derived. No doubt all these qualities would be _inconceivable_ in the absence of a mind by which they might be _conceived_, but it is not necessary that, in order to _be_, they should be _conceived_. In discussions of any abstruseness we cannot be too precise in our use of words, and we shall inevitably be