On epistemic causal że -clauses in spoken Polish

In this article, we examine subordinate causal clauses headed by the declarative complementizer że ‘that’ in Polish. By comparing them with canonical causal clauses headed by ponieważ ‘because’, we argue that causal że -clauses function as speech act modifiers. The main evidence for this claim comes from: (i) movement to the left periphery of the matrix clause, (ii) variable binding, and (iii) sensitivity to material associated with the TP domain, CP domain, and the illocutionary force of the matrix clause. Additionally, we argue that they possess distinct discursive and semantic properties, i.e., they are non-at-issue and express epistemic causality. Diachronically, we provide evidence showing that że ‘that’, as a complementizer, has undergone semantic narrowing, leading to its restricted behavior at the syntax–semantics interface in Present-Day Polish, in contrast to inherent causal complementizers.