In physics you deal with these universal laws, while, for instance, in evolutionary biology you cannot experiment with the phenomena of evolution. When you want to explain the extinction of the dinosaurs there's no experiment that you can do that would demonstrate the extinction of the dinosaurs. You have to reconstruct the past, you have to create what is called a historical narrative which is like a working hypothesis, or as Popper would call it, a conjecture. And then you test this by seeing how it fits with possible… with all the evidence that you have. And I always use as my example the extinction of the dinosaurs. At the very beginning people thought, well the dinos probably fell victim to a particularly virulent infectious disease, some pathogen that… that affected dinosaurs and nothing else, but that didn't work out, there were too many possible objections to this. And then the next theory was, well, at that time there must have been a drastic change of climate and the dinosaurs fell victim to this change of climate, to this climatic catastrophe. And that didn't work out because they couldn't find in the record of all sorts of things like sediments and… and ocean deposits and whatnot any… any indication of a drastic change of climate at that period. And finally, Alvarez came out with the theory that a meteorite or an asteroid had hit the earth and had exploded and that had been the cause of the extinction of the dinosaurs, directly or indirectly, by the change in the weather. And so, yes, a climatic change, but a very temporary one. So, here you have a narrative that Alvarez proposed and every test that has since been made fits with this scenario. In fact, they found the impact crater at the tip of the Yucatán Peninsula, and also has found the ejector from this crater in some of the nearby Caribbean islands. So, now this historical narrative proposed by Alvarez is tentatively accepted by just about everyone. And so it goes with all past phenomena. One has to… one can explain… only… let us say there is this suddenly turning up of North American mammals in South American strata at a certain time in geology and the historical narrative says well that was… was the time when the Isthmus of Panama originated between North and South America and these mammals were able to move across, walk across and reach South America. And so it is with all past evolutionary events. One must construct a historical narrative which is an inference from the existing facts and then test this narrative by all sorts of ways and means and see whether it is able to answer all the questions that are directed against it.