Which Star Trek episode is this?

I don't remember much about it, but I think there were these beings who did not have physical bodies, but only existed in a mental state. To communicate with the crew, their leader inhabited the body of Spock and said: "You humans, so many emotions. And yet so alone in your own body." It was really cool but I just can't remember which one it is! Can someone name the episode or post a youtube link?