How Science Works 37 



grouping living things on the basis of common properties. 

 For example, birds were separated from mammals by 

 having wings and laying eggs and by a number of other 

 properties. The principal point in doing this work is that 

 it simplifies our thinking about the next bird we meet. 

 As soon as we recognize him as a bird, we have a number 

 of expectations about him which are Ukely to be correct. 



Objects can be classified in a number of difl[erent ways. 

 The trick is to find the most useful one. Usually it will be 

 some simple property which a large number of objects 

 have in common and which enables us to predict a good 

 deal about how each member in the class will behave. 

 The bird example is so obvious and apparently simple 

 that perhaps it obscures rather than illuminates the subtlety 

 and power of the procedure. To get a little closer to the 

 heart of this important matter, let us look at Newton's feat 

 of abstracting the ideas of mass and force from the varieties 

 of motions observed on earth and in the heavens by his 

 predecessors. 



Even though Galileo was a very great man indeed and 

 devised most of the experiments that led up to the basic 

 ideas on which the laws of motion are based, he never 

 quite reached these ideas himself. The trouble he had was, 

 in principle, the same one that confronted us when we 

 first noticed our potato sprouts. Every event in nature is 

 the result of a large number, perhaps an infinite number, 

 of other events. Some are certainly more important than 

 others and it is the job of the scientist to find out what 

 they are and arrange them in order of importance. Just as 

 the potato sprout is conditioned by light, temperature, 

 and the availability of a number of different chemical ele- 

 ments, so the motion of an object is the complex result 

 of gravity, the motion of other bodies with which it comes 

 in contact, the resistance of the air in which it moves, the 



