-When any given bodies of the same or different magnitude are compelled by other bodies to remain in contact, or if they be moved at the same or different rates of speed, so that their mutual movements should preserve among themselves a certain fixed relation, we say that such bodies are in union, and that together they compose one body or individual, which is distinguished from other bodies by the fact of this union.
Neurri berdineko ala ezberdineko gorputz batzuk, beste batzuek tinkatzen dituztelarik elkar sostengatzeko gisan, edo mugitzen direlarik gorputz haiek, abiadura berean edo ezberdinean, beren mugimenduak neurri zehatz batean elkarri helarazteko gisan, erranen dugu orduan gorputz haiek guztiek batean gorputz bakar bat edo Izaki bakar bat osatzen dutela, beste guztietarik bereizten dena gorputzen batasun horrengatik.