Alan Kay, responding to this question on Quora:
A good enough answer would be longer than is reasonable for Quora, but I can supply a few comments to highlight just how little attention is paid in the media, histories, and by most people to find out what actually happened. For example, I was present at the visit and demo, and it was the work of my group and myself that Steve saw, yet the Quora question is the first time that anyone has asked me what happened. (Worth pondering that interesting fact!)
Steve Jobs’ famous visit to Xerox PARC to see the Alto system graphical user interface is the stuff of legend. The Mac owes its inspiration and existence to that visit. This is a great story.