DEFINITIONS. 181 



12. No material particle can exist but with time and 

 in space. Mobility is the name given to that quality of a 

 material particle by which it is made to change one portion 

 of space for another, and consequently the quantity of it 

 must depend on the space described. The velocity of a 

 particle of matter is measured directly by the space de- 

 scribed, and inversely by the time of description. Motion 

 and velocity are therefore not beings, but merely certain 

 relations which particles of matter may bear to space or 

 time. 



13. No particle of matter can pass from one moment 

 of time to another without existing throughout the inter- 

 mediate interval. Neither can any particle be moved 

 from one place to another without going through an inter- 

 mediate space. Hence we perceive the continuity of 

 time and space to have a strong influence on matter; 

 whereas the incontinuity of this has no effect on time 

 and very little on space, except so far as a greater or less 

 apparent occupation of it may be occasioned by a new ar- 

 rangement of particles. 



14. No two material particles can occupy the same 

 place at the same time ; which relation of matter to time 

 and space is termed its impenetrability/ or solidity, since 

 one particle of matter must leave the portion of space it 

 occupies before this can be occupied by another. 



15. No particle of matter can be in different places at 

 the same time ; and this property constitutes its metaphy- 

 sical identity. 



