This modern view that we have now accepted is a tremendous change from even as recent a time as the period of Enlightenment. There was a continuous change of the basic underlying ideologies. There was proposal of new things and the changes in one field, for instance, very often biology was leading and proposing changes in thinking, and they were eventually adopted also in physics. And let me take up a few of these basic ideologies… ideologies and show what they were like in the beginning and what they are like now, and the point that I make, and it's a very important one, is that the… these ideologies were not leading us to new concepts in… in science. In fact, contrary to what many historians claim, I believe that virtually all changes in science, particularly conceptual ones, came out of science, were a result of the findings of science, and were resisted by these widely held ideologies. And the greatest impediment to progress in intellectual thinking is the so-called zeitgeist, that which everybody believes. And that gets us back to [JBS] Haldane's famous eternal truths, and we are… and particularly natural selection had a great battle to overcome these widely held… generally held ideas and ideologies and break them down one by one. And let me now begin with the first one, that is, essentialism. I talked about this already before. Essentialism goes back to Plato and the Pythagoreans who said there are a limited number of classes of objects in nature; there is variation but these are only the… the incorrect shadows on the cave wall so to speak, as it was in Plato's allegory, of the real object; the real object as such can never change, it is constant. Now that essentialism, that typology, was overcome by Darwin's population thinking and is now completely… in at least in biology, completely replaced.