Saussure's linguistic theory was called structural linguistics because in his theory, and even in the linguistic theories after him, the notion of structure or ________ is of special significance.
索绪尔主张， 研究语言学， 首先是研究语言的系统（结构） ， 开结构主义的先河。 由此可知， 该空应填“系统”即system。