Modality can be expressed through a variety of different linguistic means within and across languages, of which one manifestation is through noncanonical case marking of the subject. In Ancient Greek several predicates show a systematic alternation between constructions with nominative and oblique subjects, which coincides with a difference in meaning, yielding a modal meaning in the latter case. We show how this modal meaning cannot be derived from the meaning of the individual parts of the construction, neither from the lexical material nor from the relevant grammatical elements. Instead, the data call for a constructional analysis of a modal subconstruction of the oblique subject construction, for which the modality must be attributed to the construction itself. We argue that this can be viewed through the lens of subjectification in the sense of Traugott (2003), here demonstrating that the semantic relation holding between the subject referent and the oblique case marking selected by the verb has been extended to the empathic relation holding between the speaker and his/her attitude towards the proposition uttered (Barðdal 2004). This, we believe, is how the concept of modality came to be associated with oblique case marking of subjects.