Sentence:  Economics is a messy discipline: too fluid to be a <WSD>science</WSD>, too rigorous to be an art. Perhaps it is fitting that economists' most-used metric, gross domestic product (GDP), is a tangle too. GDP measures the total value of output in an economic territory. Its apparent simplicity explains why it is scrutinised down to tenths of a percentage point every month. 
Meaning of the word marked with <WSD> tags:  A particular discipline or branch of learning, especially one dealing with measurable or systematic principles rather than intuition or natural ability. (from 14th c.)