Sentence:  Economics is a messy discipline: too fluid to be a science, too rigorous to be an art. Perhaps it is fitting that economists' most-used metric, gross domestic product (GDP), is a tangle too....But as a <WSD>foundation</WSD> for analysis it is highly subjective: it rests on difficult decisions about what counts as a territory, what counts as output and how to value it. Indeed, economists are still tweaking it. 
Meaning of the word marked with <WSD> tags:  A basis for social bodies or intellectual disciplines.