328 INTRODUCTION TO PHILOSOPHY OF SCIENCE 



only a knowledge of the basic properties of force. The pre- 

 sumption is, in other words, that certain statements about 

 force may be taken as fundamental, and others taken as 

 derivative, with the latter capable of logical deduction from 

 the former. What is demanded is a set of postulates de- 

 fining force, which will imply a set of theorems giving addi- 

 tional information about force. Then the postulate set 

 may be considered as giving the essential meaning of the 

 concept. 



In spite of very general disagreement among writers on the 

 philosophical foundations of the concepts of mechanics, 

 there is apparently a common recognition of the necessity 

 for defining force approximately in terms of Newton's three 

 laws of motion. Whether these three laws are consistent 

 and independent, and whether there is a more basic system 

 from which they may all be deduced, are questions concern- 

 ing which there seems to be some dispute. The reduction 

 of these laws to their empirical foundations is a problem 

 which, as will be seen immediately, involves some difficulty. 

 But an examination of these basic laws will afford a satis- 

 factory method for determining approximately what is to 

 be understood by force as it functions in science. 



"Every body preserves in its state of rest, or of uniform 

 motion in a right line, unless it is compelled to change that 

 state by forces impressed thereon. 



"The alteration of motion is ever proportional to the 

 motive force impressed; and is made in the direction of the 

 right line in which that force is impressed. 



'To every action there is always opposed an equal reac- 

 tion: or the mutual actions of two bodies upon each other 

 are always equal, and directed to contrary parts." x 



The first refinement to be introduced into this conception, 

 so far as modern science is concerned, is the replacement of 

 the crude notion of "body' by the more precise notion of 

 "particle." This concept was defined earlier in the chapter. 

 The essential property of particles, for the purposes of this 



1 Newton's Principia, p. 83. 



