326 INTRODUCTION TO PHILOSOPHY OF SCIENCE 



presence of matter does not help, for this places an extra 

 burden on the concept of matter. In fact, as will be seen 

 later, there is reason to believe that force is one of the essen- 

 tial features of matter and hence to be used in its definition; 

 but this would then forbid one to define force by means of 

 matter. Accordingly it may be said, for the present, merely 

 that forces are always attached to events which exhibit a 

 certain qualitative identity. Even the wind which fans one's 

 cheeks is hot or cold, fragrant or foul-smelling; and one 

 promptly materializes as 'unseen hands' any very subtle 

 forces which seem to cause him to act without effort. Dis- 

 embodied forces, like disembodied spirits, are not found so 

 long as one limits himself to the most clearly given data. 



This leads promptly to a consideration of a further aspect 

 of force — its intimate association with motion. In this 

 regard the concept of force is intimately tied up with the 

 concept of causation, to which reference will be made in a 

 later chapter. At the empirical level, force is commonly 

 defined as the cause of change in motion. Whether the cause- 

 effect relation is reducible to mere succession, or whether 

 there is something of "production" involved, are questions 

 which must be postponed. However, the feeling of force 

 seems to be an important part of all situations in which 

 one is himself active in bringing about a change in nature. 

 But that the force is the cause of change of motion rather 

 than the effect is not too obvious. When I instigate move- 

 ment, e.g., push an automobile, I feel the force before I 

 notice the motion, hence I conclude that the force has caused 

 the motion. But when I stop movement, e.g., endeavor to 

 hold a coasting automobile, I notice the motion before I 

 feel the force, hence I conclude that the motion caused the 

 force. It seems arbitrary, therefore, to say that force causes 

 motion rather than that motion causes force. What can be 

 said with a fair degree of certainty upon the empirical level 

 seems to be merely that in all situations in which there is 

 change of motion, i.e., initiation or acceleration, and stopping 

 or slowing, force is present. In other words, nature reveals 



