356 THE PHILOSOPHY OF BIOLOGY 



receivers of the wireless telegraphy apparatus). This 

 is essentially what we understand by the inertia of 

 gross matter. We set a body in motion by expending 

 energy on it (the explosion of the powder in a cartridge, 

 which converts potential chemical energy into the 

 kinetic energy of the moving projectile) ; and we can 

 stop a body which is in motion by absorbing this energy 

 of motion (by causing the projectile to strike against a 

 target, when the kinetic energy of its motion becomes 

 the kinetic energy of the heat of the arrested body) . 



Inertia is therefore the same thing whether it be 

 the inertia of visible, material bodies, or the inertia 

 of invisible, material molecules, or the inertia of the 

 immaterial, non-tangible ether. It is the condition 

 that energy-changes must occur if anything accessible 

 to our observation is to change its state of rest or 

 motion. 



ENERGY 



Energy is therefore indefinable. It is an elemental 

 aspect of our experience. 



Nature to us is an aggregate of particles in motion. 

 We have to speak of massive particles, whether we call 

 these visible material bodies, or molecules, or atoms, 

 or electrons, in order that we may describe nature. We 

 must employ the fiction of a substantia physica. We 

 only know the substance or matter in terms of energy ; 

 it is really the latter that is known to us. It is the 

 poverty of our language, or rather it is the legacy of a 

 materialistic age, that compels us to speak of par- 

 ticles that move, rather than of motions as entities in 

 themselves. 



Considering, then, the idea of particles in motion as 

 a fiction necessary for clear description, we can study 



