168 Mr WHEWELL, ON THE NATURE OF THE TRUTH 



determined what the true measure of such action was. Thus Galileo, 

 in his "Discorso intorno alle cose che stanno in su I'Acqua,'" says, that 

 momentum "is the force, efficacy, or virtue with which the motion 

 moves and the body moved resists; depending not on weight only, 

 but on the velocity, inclination, and any other cause of such virtue." 



The adoption of the phrase vis viva is another instance of the extent 

 to which men are tenacious of those terms which carry along with their 

 use a reference to the fundamental laws of our thought on such matters. 

 The party which used this phrase maintained that the mass multiplied 

 into the square of the velocity was the proper measure of the force 

 of bodies in motion; but finding the term moving force appropriated 

 by their opponents, they still took the same term force, with the 

 peculiar distinction of its being living force, in opposition to dead 

 force or pressure, which they allowed to be rightly measured by the 

 momentum generated in a given time. The same tendency to adopt, 

 in a limited and technical sense, the words of most general and fun- 

 damental vise in the subject, has led some writers (Newton for instance,) 

 to employ the term motion or quantity of motion as synonymous with 

 momentum, or the product of the numbers which express the mass 

 and the velocity. And this use being established, the quantities of 

 motion gained and lost are always equal and opposite; and, therefore 

 the quantity which exists in any given direction cannot be increased 

 or diminished by any mutual action of bodies. Thus we are led to the 

 assertion which has already been noticed, that the quantity of motion 

 in the world is always the same. And we now see how far the 

 necessary truth of this proposition can be asserted. The proposition is 

 necessarily true according to our notions of material causation ; but the 

 measure of "quantity of motion," which is a condition of its truth, is 

 inevitably obtained from experience. 



21. It is not surprising that there should have been a good deal 

 of confusion and difference of opinion on these matters : for it appears 

 that there is, in the intellectual constitution and facvdties of man, a 

 source of self-delusion in svich reasonings. The actual rules of the 

 motion and mutual action of bodies are, and must be, obtained from 



