52 THE PHILOSOPHY OF BIOLOGY 



certain entity remains constant, and no conceivable 

 process can diminish or increase its quantity. We call 

 this entity energy, and we usually extend the principle 

 of its absolute conservation to matter, though this 

 extension is unnecessary, for we must think of matter 

 in terms of energy. Stated more generally the principle 

 is that whatever exists must continue to exist, if we 

 are to regard this existence as a real one.^ 



It is not at all self-evident to the mind that energy 

 must be conserved, for we see that, to all appearance, 

 it may disappear. A golf -ball driven up the side of 

 a hill possesses energy while in flight, kinetic energy 

 or the energy of motion ; but this apparently is lost 

 when the ball alights on the hill-top and comes to rest. 

 We say, however, that it now possesses potential 

 energy in virtue of its position ; for if the hill is a 

 steep one a little push wdll start the ball rolling down 

 with increasing velocity, and when it reaches the spot 

 from which it was originally impelled it possesses 

 kinetic energy. This is described as one-half of the 

 mass of the ball multiplied by the square of its velocity. 

 Now the kinetic energy of the ball at the instant when 

 it left the head of the driver ought to be equal to its 

 kinetic energy when it reached the same horizontal 

 level on its downward roll. Yet it can easily be shown 

 that this is not the case, and we account for the lost 

 kinetic energy by saying that it has been dissipated 

 by the friction of the ball against the atmosphere in 

 its flight, and against the side of the hill on its roll 

 back. We cannot verify this quantitatively, but we 

 are quite certain that it is the case. If we take a 

 clock-spring and wind it up, the energy expended 

 becomes potential in the spring, and when the latter 



^ The principal reason why we do not beUeve in phantasms is that these 

 appearances are not conserved. 



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