Sentence:  Due to the <WSD>mediary</WSD> role of thirdness, each of the categories can intermittently play the role of any of the other categories. Yet at a given space-time juncture, one of the three will be a first, one a second, and one a third. 
Meaning of the word marked with <WSD> tags:  (semiotics) Defining or inferring a generalization or category of meaning.