40 THE ANATOMY OF SCIENCE 



pletely defined, but it is sometimes useful to 

 illustrate the meaning we desire to convey by 

 means of examples or analogies or pictures. 



It is, however, of the postulates that I wish 

 to speak. Let me remind you of their content. 



1. A straight line may be drawTi from any 

 point to any point. 



2. A straight line may be produced continu- 

 ously in a straight line. 



3. A circle may be described wdth any center 

 or distance. 



4j. All right angles are equal. 



5. If a straight line falling on two straight 

 lines makes the interior angles on the same side 

 less than two right angles, the two straight 

 lines, if produced indefinitely, meet on that side 

 on which are the angles less than the two right 

 angles. 



If after neatly packing for a journey we 

 should discover some necessary object and add 

 it as an ungainly package to our carefully 

 packed trunks and boxes, it would give the same 

 impression that Euclid produces when after the 

 rest of his simple and concise utterances he in- 

 troduces the awkward sentence that appears in 

 some editions as the last postulate, in others as 

 the last of the common notions, namely, "If a 



