Mike probably the more accurate way to put it is that these letters, plus others, and Jefferson's association with Frances Wright of A Few Days in Athens fame, shows that Jefferson understood the philosophical issues and embraced Epicurus, which is a pretty huge achievement for his time. But he was a politician more than philosopher and he kept these views largely private so there are definite limitations.
Donald's article correctly points out that Jefferson also appreciated some of the formulations of the Stoics, but what DR fails to appreciate or point out is that Jefferson did so while explicitly rejecting the core ideas of Stoicism.
To me that is consistent with a pattern I see in "modern Stoicism" to be very selective in embracing only a part of what true Stoicism was really all about. My view is that modern Stoicism is really hardly Stoicism at all, but rather a psychological therapy technique that uses the name of a philosophy to enhance its credibility.
That's why we see a steady flow of people who go looking for an alternative to religion on the internet, find Stoicism, but eventually recognize in Epicurus more of what they were looking for in the first place. Which is to a degree my own story too.
