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 The statement might be left to stand by itself, yet I think it best to explain why I use these terms. About the term unity no one will cavil.

For extension the term impenetrability has been used, but it has a negative connotation which I wish to avoid. I once thought of using dimension, but I soon found that I must use it in another sense in discussing measure. Then I thought of space. Now, space has a metaphysical use in which it is synonymous with vacuum or void and from which I wish to rescue it. So I concluded to use the term extension to signify exclusive occupancy of space, and to use space itself for the extension of positions of extensions, which also includes the extension of the medium which makes up the space. Let this be made clear. As the terms are here used the particles of the walls of this box have extension, and the particles of air which it contains have extensions, and the particles of ether within the air have extensions, but the space of the box includes the extensions of the box, the extensions of the air, and also the extension of the ether. I may speak of the space of the box and refer only to the position of the particles of the box and I may then speak of the space of the box as the sum of the extensions of the walls, air, and ether. It may be that the walls of the box have minute apertures in which air exists, so that all the air is not excluded from the wood, and it is certain that the ether is not excluded from the wood. And it may be that there are interspaces between the particles of wood, air and ether. Therefore even the wood of the box must be described in terms of space, not in terms of extension. When we come to discuss extension itself, we find ourselves considering mass, so that