[[abstract]]In modern Taiwanese Southern Min, tui displays double functions as both a SOURCE and a DIRECTION marker. The present study explores the motivations underlying the conflation of the two functions within the marker tui as well as its grammaticalization path. In the historical text Li Jing Ji, tui expresses various senses, including a noun ‘a pair’, an adjective ‘opposing’, verbs ‘to match’ and ‘to face’. In its diachronic evolution starting from the 16th century to the contemporary, tui undergoes syntactic and semantic changes. Syntactically, tui decategorizes from a full-fledged verb into a preposition. Semantically, it is argued that tui’s function as a DIRECTION marker in contemporary Taiwanese grammaticalizes from its erstwhile verbal function ‘to face’ as motivated by the force of pragmatic strengthening. Moreover, we propose that the function of marking DIRECTION further gives rise to the function as a SOURCE marker, a process which is driven by cognitive and the discourse-pragmatic motivations. In terms of cognition, the notions of SOURCE and DIRECTION are hard to differentiate without profiling. In terms of discourse-pragmatics, the motivations may be threefold. First, the collocation of tui with the spatial movement verbs provides a context in which the participant role SOURCE may need to be specified. Second, the conflation is facilitated by the operation of use-based, metonymic inferencing in language use. Third, the differentiation of the two notions is dispensable in some contexts. Along the pathway of change, tui also undergoes the processes of semantic bleaching and metaphorical extension, its original meaning ‘to face’ being weakened and the new functions of SOURCE and DIRECTION being transferred to abstract domains. It is observed that the motivation underlying the emergence of the phonologically reduced variant ui is to differentiate the two conflated roles SOURCE and DIRECTION, which manifests the prototype effect in grammaticalization. Ui arises as a specialized SOURCE marker, which allows DIRECTION to be mainly represented by tui. Therefore, the diachronic development of SOURCE and DIRECTION in Southern Min can be characterized as undergoing both the processes of conflation and differentiation. In Li Jing Ji, SOURCE and DIRECTION are distinguished by separate markers. In contemporary Taiwanese Southern Min, however, the two roles are conflated within tui. The conflation is succeeded by the process of differentiation, as tui begins to be specialized as a DIRECTION marker and ui a SOURCE marker.