Science and Policy 205 



ences of opinion on purely scientific matters when new 

 procedures are introduced into practice. The reason is that 

 one wants to enjoy the benefits of the new procedure as 

 soon as possible. If one waited for complete agreement 

 on the exact probabiUties, one might have to wait many 

 years. 



Even when we have a clear picture of the mathematical 

 probabilities, our troubles are not over, for now we en- 

 counter a value problem in addition to the purely scientific 

 problem discussed above. Let us say, for example, that 

 there is a disease that kills one person out of a thousand, 

 and we have in our hands a vaccine which we are pretty 

 sure confers immunity on 95 percent of the people we 

 give it to. Other things being equal, it should reduce the 

 mortality rate from, one in 1,000 to one in 20,000. But 

 unfortunately we know that the vaccine may itself produce 

 a fatal case of the disease in one out of 10,000 people. It 

 appears that if we give our vaccine to a million people we 

 will save 950 people who would otherwise have died from 

 the disease and bring about the deaths of 100 other people 

 who might not have died, leaving us with a net gain of 

 850. In objective, cold-blooded terms, it looks like a good 

 trade, but the fact of the matter is we don't do it. The de- 

 cision grows out of the way we settle the value question. 

 In its crudest terms it appears that it is morally and ethically 

 preferable to allow 900 people to die by doing nothing 

 than to contribute to the deaths of 100 people by doing 

 something. Most of us instinctively agree with this assess- 

 ment and it has the sanction of Hippocrates' ancient in- 

 struction, "In the first place, do no harm." 



Nevertheless, we do not push the conclusion to its ab- 

 solute limit. Again, most people instinctively agree that 

 there comes a point at which the odds convince us that the 

 risk is worth taking. For example, we regularly urge every- 



