But McCarthy's legacy also has a little-noted dark side which also influenced my career, but in a much less positive way. McCarthy was not only one of the pioneers of the study of AI, but also an avid proponent of a particular school of thought about how human intelligence works. McCarthy believed that human intelligence could be modeled as a formal logic. That hypothesis turns out to be (almost certainly) wrong, and the evidence that it is wrong was overwhelming even in McCarthy's heyday. And yet McCarthy steadfastly refused to abandon this hypothesis. Well into his nominal retirement, and quite possibly to his dying day, he was still working on trying to formulate formal logics to model human thought processes.
The way human mental processes actually work, it turns out, is (again, almost certainly) according to statistical processes, not formal logics. The reason I keep hedging with "almost certainly" is that the jury is still out. We have not yet cracked the AI puzzle, but vastly more progress has been made in recent years using statistical approaches that has ever been made using logic. Very few (if indeed any) logic-based systems have ever been successfully deployed on non-toy problems. Statistics-based applications are being deployed on a regular basis nowadays, with Siri being the most recent example.
It took decades to make this switch, arguably due in no small measure to McCarthy's influence. One of the many consequences of this delay was the infamous AI-winter, which lead more or less directly to the commercial demise of Lisp. That the same person was responsible both for the invention of such a powerful idea and for its demise has to be one of the greatest ironies in human intellectual history.
It is important to remember that even great men can be wrong at times, sometimes spectacularly so. There is no shame in this. The human has yet to be born whose rightful epitaph is "He was right about everything." But John McCarthy's legacy in particular calls all of us mere mortals to a greater degree of humility. The world would be a better place if more people could acknowledge the possibility that even their most cherished beliefs might be wrong.
[UPDATE:] After posting this I felt the need to hedge my assessment of Lisp as "mostly dead." Lisp is not dead. In fact, it is probably more vibrant now than at any time in the last 20 years. But by comparison to other languages Lisp has a vanishingly small mindshare. To cite but one concrete example, of 300 or so Y Combinator companies there is (AFAIK) only one whose code is written in Lisp.
Notwithstanding all that, if you're interested in programming I really encourage you to learn Lisp. It is still the best programming language out there.