just adding a few more words to complete the story.
after so much theory, a new trend has emerged in the chess elite. this is to reject all principles and to trust nothing but concrete analysis. the adherents of this new world view are willy hendriks and john watson both of whom have written books dedicated to this very subject, the book of willy hendriks having received the british chess federations book of the year award. it is the contention of hendriks that principles really do not matter, only the moves and the way to improve is to force feed yourself with lines and with as many types of positions and moves as possible. hence the title of his book 'move first, think later.' also worthy of metnion is jonathon rowson and his book 'chess for zebras'-excellent title and great book.
I have often wondered if we play the we moves we play because of tradition or because they are the best moves. Havening recently delved into Kasparov's great predecessors book I have come to the realization that the lines we play are much like the evolution of automobiles. First, someone invented the wheel, so these were probably the oldest players like philidor and lucena who made the first observations, then some people improved, sort of like sharpened the blade, enter morphy and the romantic era of open games, importance of the center, importance of development so on. It seems to me that every opening has a history much like a science which continues to change and evolve to the present day. So has playing style changed down the centuries but that is a secondary matter. The amount of refinement and discovery has been slow, like the growth of a science or like the changes in the style of paintings with newer artists improving on the masters of the past. Some of the lines like pelikan eveolved from earlier lines and likewise gave rise to newer variations such as the chelyabinsk which even magnus plays today. then again some lines came and quickly went out of fashion whereas some others were beaten to death like the guioco piano after centuries of improvement. the ruy lopez one of the first openings to be discovered sprang off variants as late as the time of marshall and his famous marshall attack. the versatile sicilian defense, one of the most famous openings of all times was injected with new life with najdorf's discovery 6.. a6!. Even after that even as late as the 70s this process continued, fischer coming up with the bc4 line, the poisoned pawn variation sometimes rising and sometimes falling out of favor due to discoveries and refutations. even today, this line continues to evolve. in conclusion, the lines which are played today are the product of centuries of trial by the best minds. and still the process of elimination and new discoveries continues.