Paraphrased from George Philander:
The hierarchy of science: Mathematics atop Physics atop Chemistry all the way down to the social sciences, is also an inverse hierarchy of simplicity. The problems which can be simply described and for which there can be no disagreement lie at the top, and the most difficult ones, the ones which permit a type of field-wide dissonance lie at the bottom. The most complicated and most relevant problems are those ones which are the least "pure"
This hierarchy of purity was once cartooned by Randall Munroe of XKCD:
Oceanographer, Mathemagician, and Interested Party