EXAMINATION OF THE PRINCIPLES 157 



cal laws, and regards them as supported only by approximate 

 verifications; nevertheless it maintains that they are universally 

 and exactly true. They are experimental laws exalted by con 

 vention into absolute principles; and they owe their security to 

 the fact that they cannot be submitted to any decisive experi 

 mental test, because they apply perfectly only under conditions 

 which surpass the limits of possible observation. 1 The theory 

 certainly is not without attractiveness. According to the first 

 law of motion, a body under the influence of a single force moves 

 in a straight line. No body, however, can be pointed out that 

 meets this condition. On the contrary, the forces acting upon 

 every body within our notice appear to be unlimited in number. 

 To find an exact application of the law we should be forced to 

 imagine a body at an infinite distance from the rest of the uni 

 verse. Similarly, the law of the conservation of energy involves 

 the notion of a closed system, a notion for which no corresponding 

 object can be found except the universe as a whole. So, too, in 

 the case of the lever. The formula which describes its action 

 requires that its fulcrum be a mathematical straight line, a con 

 dition which we find nowhere realized. The endeavor to find the 

 perfect lever simply leads us to dissect the visible lever into 

 smaller and smaller segments, without a real expectation of ever 

 arriving at a satisfactory conclusion. 



But now let us ask whether it is in such application as this that 

 the significance of these laws really consists? Do we not see 

 the realization of the first law of motion in the missile which is 

 thrown from the hand? To be sure, neither is it acted upon by a 

 single force, not is its course a straight line. But we say that 

 in so far as other forces are negligible in comparison, the course 

 is indeed a straight line. And do we not find the law of the con 

 servation of energy realized in the relation of the food we eat 

 to the work we do? Here again, to be sure, there are other 

 sources of energy and abundant avenues for the escape of the 

 energy developed; but even in so crude an example as this, we 



l Science and Hypothesis, pp. 98-100. 



