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 mass and extension are here nearly synonymous; but mass is used as the measure of extensions, while space is the dimensions of related positions. Mass is the measure of the numbers of particles of extension, but units of space are measured with units of length.

I use the term speed because in modern physics it has exactly the meaning which I desire. The popular meaning of velocity is just what I need, but in physics velocity means rate of speed and also rate of deflection and the term is needed for that purpose.

I use the term persistence because the term time or the term duration means persistence and change or they may mean the measure of states separated by change, while the term persistence is free from these implications.

If the terms are understood we are ready to proceed to another stage of exposition.

Essentials are comprehended in the same particle, and we shall call them concomitants. We shall not say that one essential is related to another in the same particle, but they are concomitant with one another, though the essential of one particle may be related to the essential of another particle. A unit may be related to another unit, an extension may be related to another extension; but the unit and the extension in the same particle are not related to each other but concomitant with each other, and these same distinctions must be observed with all the essentials. The task before us in this chapter is the exhibition of the concomitants of particles and relations of essentials, concomitants inhering in every particle, the relations arising by reason of the relation of particles to particles.