I've always understood subtle to mean the opposite of obvious, as in subtle difference, until I came across this line from The Duchess of Malfi
Oh, sir, the opinion of wisdom is a foul tetter that runs all over a dead man's body. If simplicity direct us to have no evil, it directs us to a happy being. For the subtlest folly proceeds from the subtlest wisdom.
I had to revisit a dictionary to check for other possible meanings because I couldn't make any sense of wisdom that's barely noticeable. Now it seems to me that subtle here is used in two different meanings: subtle folly is folly that can barely be seen, and subtle wisdom is wisdom that's deep and penetrative. So, the way I understand the sentence is if you have deep wisdom, you're least likely to experience folly, and vice versa. How correct am I?
The Webster dictionary has this one sense among others for subtle:
Having or marked by keen insight and ability to penetrate deeply and thoroughly: a subtle scholar.
So I thought this was probably the right sense for subtle wisdom. Then I ran a google search with "his subtle wisdom" (with quotes), and finally landed on the following quote from this website:
The premise for each episode was simple, Barry Livingston says. “The boys have a problem, and MacMurray, with his subtle wisdom, lets them work it out—but guides them with an invisible hand.”