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My first-shot recommendations for the five philosophical works an educated person should have read:
Plato, Gorgias
Mengzi
Marcus Aurelius, Meditations
Boethius, Consolation of Philosophy
Hume, Enquiry Concerning Human Understanding
My thinking is that all of these are texts
(1) of considerable actual historical influence
(2) that are more or less universally accessible to those willing to put in the effort
(3) that do not require extraordinary assistance for most people to read, as long as they put in the time and effort
(4) that model how to do philosophy in a way that can be fairly easy to follow for most careful readers
(5) and that touch on topics of more or less universal interest
The trade-offs are interesting in something like this. I think a number of the big works in philosophy tend to be next door to incomprehensible even to most intelligent people without considerable help or a lot of preparation, while some of the most accessible works are marginal historically; and I think some works that are obvious candidates if you are teaching Intro are not necessarily great candidates if we are considering educated people who are interested in learning what it's about on their own time.