If I have to go back to school and master a subject, that would be Psychology. I have always been very curious about humans, what makes them tick, how and why we react a particular way, the emotions we feel… etc. etc.
I am fond of Arts, but got caught up with Science, Computer Science specifically. But in this line, the one subject that I would like to master, is Artificial Intelligence, which has close relationship with Psychology.
And here is a photography of a Mastery at work (Location: St. Peters Basilica).