Ha! Being obvious to you does not mean you truly understand it.
At the start of my PhD my argument was obvious to me, but it's taken many years of hard work to turn that implicit understanding into an explicit, proper understanding (and I'm not fully there yet).
The more I've learned and studied various areas, the more I've come to realize that true mastery is to really, really understand the 'basics' of a given field. If you can really understand those foundational aspects of a field, all else can be derived almost through intuition. Unfortunately it takes a few years to realize that you don't in fact know the basics, and many more years to really 'get' them ;)
I think that hints at what I would consider a more useful criterion for true understanding: being able to apply something in new ways. Any programmer can learn to implement a linked list by being beaten over the head with it in a data structures class until it seems obvious. Someone who truly understands it will be able to pull out concepts like pointers from it and reuse them when confronted with another problem, like making a hash table that can handle collisions.
At the start of my PhD my argument was obvious to me, but it's taken many years of hard work to turn that implicit understanding into an explicit, proper understanding (and I'm not fully there yet).