Freedom & Oxygen (and carbon, hydrogen, nitrogen)
Ideas can give complex topics a unity. An example from history is the contradictory interpretations of "freedom". In nature, an example (the most basic?) is the scientific discovery of the elements of life being present in the stars.
Freedom: In the US, how did slave owners write so poetically of all men being free? Compare the early 20th century phrase "wage slavery" to the late 20th century argument that the economy needs to be "free" of government interference. The idea that US history can be examined via different meanings of the word "freedom" gave me a way of unifying it, though there are other themes that can be used. (Inspiration: the books The Story of American Freedom and the stimulatingly different Patriots History of the United States, among others).
"Freedom" could be seen as the "oxygen" of democracy, but that's not where I'm going today.
Compared to "freedom", the discovery that all plants and animals live by carbon, hydrogen, oxygen, and nitrogen is more fundamental, all the more so with the more recent discovery that the same four elements are "star stuff". Biology and astronomy somehow make more sense to me now. (My limited knowledge of this is mostly from the book The Scientists by John Gribbin).
I'm not saying that US history is reducible to the above (and many more) usages of the word "freedom", nor am I suggesting that biology and astronomy are simply made of carbon, oxygen, hydrogen, and nitrogen. In US history there are varying interpretations and other themes that could be used, in the sciences the details are complex. But these two basic ideas did give me a new perspective, a new way of viewing the topics, a unity, or theme, in complex subjects.
Is there a big human idea that inspired you to see a topic of study differently? Another field with a leanse which is one of many? Or a complex field with a fact that is discovered to be fundamental?