diatheesis

Diathesis, sometimes spelled diatheesis in older texts, is a linguistic term that denotes the relationship between a verb and its participants—the verb’s valency—and how those participants are realized in syntax. It describes how the event described by a verb is framed in language, including who is performing the action, who is affected, and how the relationship between these roles is encoded in a sentence.

In practice, diathesis is most often discussed in terms of voice. The active and the passive are the most familiar diatheses: active foregrounds the agent, as in “The chef cooked the meal,” while passive foregrounds the patient or theme, as in “The meal was cooked (by the chef).” Beyond voice, many languages employ additional diatheses: middle or reflexive (the subject participates in the action in a way that affects itself, as in “The door opened” in some contexts), antipassive (valency reduction, often downplaying the object), causative/inchoative (changing the number or status of participants), and applicative or other constructions that add or focus on a beneficiary or goal. Examples vary by language; English provides clear active and passive pairs, while other languages illustrate these alternations more extensively through morphology or special verb forms.

Diathesis is a central concept in syntactic theory and typology because it links syntax, semantics, and morphology through the study of how languages encode event structure and argument realization. It helps describe cross-linguistic variation in how many participants verbs require and how they are expressed in different constructions.