哲學家S. E. Marshall論證「性應該是相互的」，並且由此推論「性交易 ( 賣淫 )因為不是相立的性，故而是錯的」。本文則從理論與例證反駁了Marshall的論證，並且釐清了「相立性」或「立惠性」的意義。

Philosopher S. E. Marshall argues that sex should be mutual, and thus sex trade is wrong because its mode of sex generally lacks mutuality. This essay anaiyzes and refutes Marshail’s argument by using various examples and showing mutuality sometimes may be incompatible with reciprocity.