One bit of magic is covered in the Law of Equivalent Exchange. In order to gain something, you have to give up something of equal value.
OK, that's covered in most of the courses, at least if the teachers know what they're doing. However, there was a very interesting take on this principle in a recent episode.
Edward, the older of two brothers, encountered a man who had lost his leg in the war. Edward, who had lost his arm and leg while attempting some serious alchemy, had mechanical replacements. He asked the man why he hadn't acquired a mechanical leg to replace his missing one.
The man cited the Law of Equivalent Exchange, and told Edward that after losing that leg, he emerged with a great sense of peace, and wasn't willing to give that up just so he could walk without crutches again.
The Law of Equivalent Exchange states that in order to gain something, one has to give up something of equal value.
It also implies that in order to lose something, one has to gain something of equal value. The trick here is, if you're too focused on what you've lost, you may never realize you have a gift, sitting right there in front of you.