I may be the last one to get the joke here, so excuse me if that is so. Marcel enters the Guermantes courtyard and trips over an uneven paving stone. He is flooded with a happy sensation, one that he quickly traces to standing on the uneven floor of the baptistery of St. Marks in Venice. The joke: John Ruskin, who Proust translated, was the author of The Stones of Venice, a study of Venetian architecture. Proust dedicates this definitive passage on unforced memory to Ruskin.
Revolving the gloomy thoughts which I have just recorded, I had entered the courtyard of the Guermantes mansion and in my absent-minded state I had failed to see a car which was coming towards me; the chauffeur gave a shout and I just had time to step out of the way, but as I moved sharply backwards I tripped against the uneven paving-stones in front of the coach-house. And at the moment when, recovering my balance, I put my foot on a stone which was slightly lower than its neighbor, all my discouragement vanished and in its place was that same happiness which at various epochs of my life had been given to me by the sight of trees which I had thought I recognised in the course of a drive near Balbec, by the sight of the twin steeples of Martinville, by the flavour of a madeleine dipped in tea, and by all those last works of Vinteuil had seemed to me to combine the quintessential character. Just as, at the moment when I tasted the madeleine, all anxiety about the future, all intellectual doubts had disappeared, so now those that a few seconds ago had assailed me on the subject of the reality of my literary gifts, the reality even of literature, were removed as if by magic.
…Every time that I merely repeated this physical movement, I achieved nothing; but if I succeeded, forgetting the Guermantes party, in recapturing what I had felt when I first placed my feet on the ground in this way, again the dazzling and indistinct vision fluttered near me, as if to say: “Seize me as I pass if you can, and try to solve the riddle of happiness which I set you.” and almost at once I recognised the vision: it was Venice, of which my efforts to describe it and the supposed snapshots taken by my memory had never told me anything, but which the sensation which I had once experienced as I stood upon the two uneven stones in the baptistery of St Marks’s had , recurring a moment ago, restored to me complete with all the other sensations linked on that day to that particular sensation, all of which had been waiting in their place–from which with imperious suddenness a chance happening had caused them to emerge–in the series of forgotten days. (VI,255-256)