Did you know that the broadcast order of Star Trek: The Original Series is not the order in which they were produced? Gene Roddenberry was an adept of certain mystery schools of religion and intentionally encoded various of the doctrines thereof in his storytelling through this series. He did so effectively and at times so blatantly, the executives of the studio chose to air them in a different order so as to obscure his intent—Season 2, Episodes 1-4, are a real eye opener!
If you're as much of a fan of this series as am I and you haven't done so yet, why don't you do a rewatch in the original production order? To my knowledge, no service provider has ever made the episodes available in the production order, so to enable you, my dear reader, to do so more easily and readily, here is the list. It's a little bit of work to keep track of one's progress but the reward is well worth it. Enjoy!
Episode Production Order