Paul Morphy's Rating>2638

The answer seems to me to be that we are simply not in a position to know whether or not Morphy would compete in the modern world and how he would rate if he did. What reason is there to suppose that any rating would not depend greatly on the circumstances of the competition?

That is what is being debated ... Is Morphy worthy of the 2638 rating? YES, he easily crushed the top competition of his day and the greatest GM of all-time Bobby Fischer agrees --or-- NO, he played positionally weak moves, does not have the theoretical or strategic knowledge of a modern barely-master (a BM... get it?), and Fischer is a nut-bag.

too big a gap with modern chess theory. the jump from morphy to lasker is BIG. this is not to say morphy coudnt beat him ,but the way chess was played was reaching its turning point. i dont think morphy could have held his own vs capa the way lasker having absorbed the teachings of steinitz did. forget modern players.

if you look at chess history, you will often see that many of the top players where a positive influence on each other. their rivalry often pushed their game to the next level, (and not just the top-top players, but  those lesser rivalries as well). Lasker has Steinitz, and then Capa to keep him in shape, Fischer had Petrosian, spassky and to a lesser degree Larsen, Karpov and Kasparov each other and so on.

who did morphy had? he had some games vs anderssen and zukertort but Morphy was better than them at their own romantic game and had no rivalry with Steinitz to move his game to the next level. he was too ahead of his time, as far as talent is concerned, but no one from the  coming age to develop with.

To clear up a few points:

Morphy never played Zukertort

Lasker's rivals were more Pillsbury and Tarrasch

But to contribute--your last point is my main point.  Throughout his life Morphy had no one to test him, to bring out the full depth of his play.  It seems the only time he worked on his game (the way we would today describe working on your game) was during the Harrwitz match for two nights between games 2 and 3--that's it.  Look at how far he got without the work, without being pushed.  Had he been pushed I'm sure there was another level to his play, but he was never pushed, so we'll never know.

That coincides with this passage from Lawson's book

[Lawson] "March 15, 1873 "...a letter from Charles J. Woodbury to the Hartford Times disclosed that Morphy still played chess, but only on special occasions and in privacy, although this time it was a "numerous" privacy, so to speak.... :

     'A flight of stairs leads the way up to the dwelling-rooms. I had never seen Paul Morphy, but I knew him the moment he stood quietly before me, simply dressed, slight, smooth and melancholy-faced, with a head and brow over-hanging with their own weight. So full of dignity, so empty of self-consciousness, was his presence, that I was almost prepared by it for the quick answer he made me that he was but an amateur, and was adverse to notoriety. But the passion of the Creole eyes overspoke the tutored voice at a remark I made about the contrast between what he said and what he had done. My imperfect French added to the embarrassment of the moment, and his thin self-control gave way to one of those paroxysms of passion to which I have since learned he is constantly subject. Happily, the coming of his mother soon divested him of the strange suspicion that I thought him to be a professional gambler; and, afterwards, through Mons. C. A. Maurian, an intimate friend and the best public player in New Orleans, all of these misunderstandings were removed...
     Once in a while, the solitary athlete can be induced to show that his power is only in abeyance. I saw him at a private séance, just before I left, beat simultaneously, in just 2¾ hours, sixteen of the most accomplished amateurs in New Orleans. His strength had never been fully tested, and will probably never be fully developed.'"

yes, i did mess up with zukerkort lol , as for lasker, its quite telling that an Capa still held the elderly Lasker as very dangerous. i dont think Capa would have had nearly as much trouble vs morphy.

And there lies the rub... Morphy was so strong that virtually everyone admits ignorance as to how strong he really was or could be.  In other words, the biggest argument against Morphy's supreme playing strength is Morphy's supreme playing strength. 

Morphy's major chess play was against 1857-8 opponents. It has to be considered whether or not that is sufficient to get an idea about how Morphy would perform against players today (and how that performance would depend on the amount of modern preparation he would be allowed to have).

"Lasker ... didn't understand positional chess." - another Fischer quote from around the same time as his Morphy comments.
Extended discussions of Morphy have been written in books by GM Franco, GM Beim, GM Ward, GM Marin, GM Bo Hansen, GM McDonald, Garry Kasparov (with Dmitry Plisetsky), and GM Gormally. Anyone see any of them express the view that we should accept Fischer's conclusion about Morphy? There seems to be general agreement that Morphy was, as GM Fine put it, one of the giants of chess history, but that is a long way from saying that he was better than anyone playing today.

that was shown against 1857-8 players.

You mean Steinitz' contemporaries?  He and Morphy had many common opponents.  Remember that Steinitz as a geriatric citizen did well against a young dominant champion Lasker ... Steinitz, in fact, did better against Lasker than did Tarrasch or Marshall in their matches, and Lasker didn't really fall behind Capa until around time of World War I, and was still dangerous into the 1930's.  Now we are talking about Morphy's inferred playing strength based on succession of high-level players nearly 75 years after his original dominance in the 1850's-60's, and this in turn is based on the games Morphy actually DID play against contemporaries of Steinitz, not the strength he could have achieved had he not been so dominant against the rest of earthlings. 

No, the major success of Steinitz was about two decades after the major success of Morphy. How much did the chess world change during that time?

Morphy and Steinitz were roughly the same age ... do you really think that Steinitz and all of his (and Morphy's) contemporaries could understand and learn chess better than Morphy who was the same age and already light years ahead them all far before his potential peak when he quit in his mid-twenties? Why wouldn't Morphy be able to handle or learn what the rest of his inferior --nay, far inferior-- contemporaries did?  Do you think Morphy peaked in 1858 and was on the decline at age 22 while Steinitz had this sudden surge of inexplicable genius into his forties? 

dannyhume wrote: "Morphy and Steinitz were roughly the same age …"

But the Steinitz major successes were about two decades (or more) after Morphy's major successes.

At the time of the major successes of Steinitz, the chess playing population was very different from that of 1857-8.

At the time of the major successes of Steinitz, I see no reason to doubt that his opponents were better than those players faced by Morphy in 1857-8.

I see no reason to believe that we know how Morphy would have done against the opponents of Steinitz at the time of the major successes of Steinitz.

We don't know what would have happened during the two decades after 1857-8 because, for the most part, Morphy chose not to be involved with the serious chess world during the two decades after 1857-8.