26 SCIENTIST 



bers. Modern probability theory seems to have been started 

 by a Hvely and intelHgent ItaUan physician * who spent 

 much of his time gambhng. In an effort to improve his 

 returns, he invented methods for calculating the odds in 

 various popular card and dice games, and the develop- 

 ment of probabihty theory has been closely related to gam- 

 bhng ever since. A very sophisticated modern branch of 

 such studies is actually named "Monte Carlo theory." 



We cannot go into all the details of probability theory 

 in this book. Suffice it to say for our purposes that it is 

 now customary to express the probability that something 

 will happen on a scale which extends from zero to one. 

 Something that never happens is said to have a probabihty 

 of zero. Something that always happens has a probabihty 

 of 1. Something that happens half the time, like a penny 

 coming down heads, has a probabihty of 0.5. Strictly 

 speaking, the two ends of the scale are regarded as limits 

 and, since we can't be absolutely sure about anything, it 

 is customary to say that the probability of an event like 

 the rising of the sun is very close to 1, or of the occur- 

 rence of a yellow fever epidemic in New York is very 

 close to zero. Much more use is made of the intermediate 

 parts of the scale; for example, the probabihty that a baby 

 born in the city of New York will die before the end of 

 one year is approximately 0.027. 



One of the major uses of probability theory is to tell a 

 given scientist and the colleagues he wishes to communi- 

 cate with how sure he is that he is right. There are two 

 kinds of problems here. The first is the relatively simple 

 matter of asking how sure we are that some elementary 



* Gerolamo Cardano (1501-1576). Because Cardano's work was not 

 published until much later and exerted relatively little influence, credit 

 is more usually given to a French nobleman, Chevalier de Mere, and 

 the philosopher and mathematician, Pascal. Cf. Lady Luck by Warren 

 Weaver, Doubleday & Company, Inc., Garden City, 1963. 



