354 THE PHILOSOPHY OF BIOLOGY 



It is heavy, and the degree of heaviness is proportional 

 to the quantity of matter in the body which we feel, 

 that is, to its mass. Heaviness is s3monymous with 

 weight, but weight does not depend alone on the quantity 

 of matter in the body. If the latter were removed to an 

 infinite distance from the earth or other cosmic bodies, 

 its weight would disappear, but its mass would remain. 

 We could still touch and move it, and we should still 

 find that different degrees of muscular exertion would 

 be necessary when bodies of different masses had to 

 be moved. 



INERTIA 



If the body were in motion, we should find that 

 muscular exertion is necessary in order that it might be 

 brought to rest ; and if it were at rest, we should find that 

 muscular exertion was necessary in order that it might 

 be moved. The body, matter in general, possesses 

 inertia, and this is its most fundamental attribute. 

 Mass we can only conceive in terms of inertia. If 

 two bodies were at rest, and if the same degree of 

 muscular exertion conferred on each the same initial 

 velocity of motion, their masses would be equal. If 

 the same degree of muscular exertion conferred diffe- 

 rent velocities on different bodies, their masses would 

 be different, and would vary directly with the initial 

 velocities conferred. 



FORCE 



The feeling which we experience when we move a 

 body from a state of rest, or stop a body which is 

 moving, is what we call force. If on climbing a stair in 

 the dark we think there is one step more than there is, 

 and so have the queer, familiar, feeling of treading on 



