Rumsfeld made this truth common sense, yet not very common still.
There are things we know but we do not know we know
There are things we know we do not know.
There are things we think we know yet we do not know
There are things we do not know we do not know
Always there are things we do not know; there are things we do not know because we cannot know them, there are things we do not know but we can know or understand them when presented to us or when we seek them. Still there are things we can know but we do not have the ability to accommodate everything in our mind, so we choose what we should know and what we should not. Life entails searching for things we can know, master them and put them in use.
Unhappiness sets in if we give our life to the search of things that are beyond human knowledge (like who created God). Happiness in life involves knowing what can be known, then decide whether there is value in knowing it or not, if there is value in knowing then master the unknown and make use of your knowledge.