From "Gunsmoke: New Doctor in Town (#17.5)"
[reading Doc's farewell letter
Living in Dodge has been the most rewarding experience of my life, but I've been so out of touch with modern medicine that when I arrived in Baltimore I felt like a schoolboy. I suddenly realized how little I know about medicine and how much has passed me by in the last twenty years... and that's why I've decided to stay on here at the university.
[Kitty, overcome with emotion, hands the letter to Newly to read
This letter is the only way I know of to tell you my decision. I'll be gone from Dodge for a while. I don't know for how long, but it will not be forever. I will be back... and I'll think of you very often. I pray all of you will remember with affection, your old friend, Doc.