Any scientist who is not a hypocrite will admit the important part that luck plays in scientific discovery. Our estimate of the importance of luck is inherently biased: we know when we benefit from luck, but in the nature of things cannot assess how often bad luck deprives us of the chance of making what might have been an important discovery.

A colleague and I carried out an experiment in which little tissue fragments, which were very difficult to work with, were injected into mice of different strains. If we had been more experienced, we would have injected only white blood cells (which would have been easier to handle) into the mice. We now know that if we had done this, we would not have discovered actively acquired tolerance because the grafts would have in effect rejected their hosts. Obviously, we were lucky, but our scientific training enabled us to recognize the significance of the accident. I think, therefore, that there was no need for the distinguished neurophysiologist Hodgkin to refer to his "feeling of guilt about suppressing the part which chance and good fortune played in what now seems to be a rather logical development."

It might nevertheless seem as if luck plays a dominant role in scientific discovery. I would like to challenge this view for the following reasons: we sometimes describe as "lucky" a person who wins a prize in a lottery at long odds; but if we describe the accidental discovery on a park bench of a lottery ticket that turns out to be the winning one?

The two cases are quite different. A person who buys a lottery ticket is putting himself or herself in the way of winning a prize. This individual has, so to speak, purchased candidacy for such a turn of events and all the rest is a matter of mathematical probabilities. So it is with scientists. A scientist is anyone who, by observations and experiments conducted, by the literature read, and even by the company kept, puts himself or herself in the way of making a discovery. These individuals, by deliberate action, have enormously enlarged their awareness—their candidacy for good fortune—and will not take into account evidence of a kind that a beginner or a casual observer would probably overlook or misinterpret. I honestly do not think that blind luck of the kind enjoyed by someone who finds a winning lottery ticket for which he or she has not paid plays an important part in science or that many important discoveries arise from the casual intersection of two lines.

Nearly all successful scientists have emphasized the importance of preparedness of mind, and I want to emphasize that this preparedness of mind is worked for and paid for by a great deal of exertion and reflection. If these exertions lead to a discovery, then I think it would be inappropriate to credit such a discovery to luck.