Q. Please tell me if it’s permissible to use a comma rather than a semicolon in the following sentence: “The idea isn’t to use the test to get people in trouble, it’s to help them avoid decisions they’ll regret later.” The rules seem to suggest that a semicolon is preferred but not absolutely required; a semicolon feels to me like it separates the thoughts more than I’d like.

A. Two independent clauses not joined by a conjunction are best separated by a semicolon or a period. This principle has always been the fifth major rule put forth in The Elements of Style—starting with Strunk’s original (Ithaca, NY, 1918) and continuing almost unchanged (“Stevenson’s romances” are exchanged for “Shelley’s works” in one of the examples, possibly for reasons of political correctness) through the latest (fourth) edition of Strunk and White (New York, 2000). If your example isn’t an exception to this rule, it probably should be. It’s a variation on the not . . . but construction:

The idea isn’t [or is not] to use the test to get people in trouble but to help them avoid decisions they’ll regret later.

Because your example echoes that form so closely, separation of the negative and positive clauses by a semicolon or period does seem a bit stilted. In fact, the difference between the two clauses seems to be made stronger by opposing them to each other more closely. Sometimes the ear is more important than the rulebook.