The whole passage: "Physicists like to think that all you have to do is say, ‘These are the conditions, now what happens next?’ But all our sister sciences have a completely different problem: in fact all the other things that are studied — history, geology, astronomical history — have a problem of this other kind. I find they are able to make predictions of a completely different type from those of a physicist. A physicist says, 'In this condition I’ll tell you what will happen next’. But a geologist will say something like this — 'I have dug in the ground and I have found certain kinds of bones. I predict that if you dig in the ground you will find a similar kind of bones’. The historian, although he talks about the past, can do it by talking about the future. When he says that the French Revolution was in 1789, he means that if you look in another book about the French Revolution you will find the same date. What he does is to make a kind of prediction about something that he has never looked at before, documents that have still to be found. He predicts that the documents in which there is something written about Napoleon will coincide with what is written in the other documents. The question is how that is possible…"