On The Road Ch. 4-10

p. 26 "Big Slim and I spent many nights telling stories and spitting tobacco juice in paper containers.  There was something so indubitably reminiscent of Big Slim Hazard in Mississippi Gene's demeanor that I said, "Do you happen to have met a fellow called Big Slim Hazard somewhere?"  

And later on p. 27 "And that was exactly right; and I still couldn't believe Gene could have really known Slim, whom I'd been looking for, more or less, for years."

Meeting new friends during your travels who remind you of past friends, and who actually know of this past friend definitely adds to a good, memorable road trip.  This scene in the book proves that road trips are full of surprises, unexpected and pleasant surprises.  Not only does a road trip encourage you to look forward to your destination and live in the present, a good road trip makes you reflect upon the past.  

Question:  What is meant by Sal's statement, "That last thing is what you can't get, Carlo.  Nobody can get to that last thing.  We keep on living in hopes of catching it once for all."?