In the course of writing about natural kinds, I approached Hilary Putnam with a specific question about his use of the phrase. He took the occasion to reflect on connections between Mill's Logic and his own semantic externalism.
I made a passing reference to the correspondence in a paper at the time. Years later, in case the letters are of any interest, I posted them in full.
In addition to providing some insight into the philosophical issue, the letters document what a nice guy Putnam was. His replies were prompt and generous, even though he didn't know me outside of this unsolicited e-mail contact.