Ancient Indo-European verbal syntax, as attested in early Vedic Sanskrit, exhibits numerous examples of the labile syntactic pattern: several verbal forms can show valence alternation with no formal change in the verb; cf. pres. svadate 'he makes sweet' / 'he is sweet'; perf. vavrdhuH ~ 'they have grown' (intr.) / 'they have increased' (tr.).
It is argued that the labile patterning of the Vedic verb, however common it may appear, is mostly of a secondary character. There are a limited number of reasons which give rise to labile syntax: (i) the polyfunctionality of the middle inflection (which can be used to mark the anticausative, passive and reflexive functions, on the one hand, and the self-beneficent meaning of the transitive forms, on the other); (ii) the homonymy of some middle participles "shared" by passive (medio-passive aorist, stative) and nonpassive formations; (iii) the syntactic reanalysis of intransitive constructions with the accusative of parameter/scope (content accusative) as transitive-causative. As to the perfect, it could probably be employed both intransitively and transitively already in Proto-Indo-European, although the intransitive usages were prevalent. In the historical period the newly-built perfect middle forms have largely taken over the intransitive function, but active perfects are still quite common in the (more archaic) intransitive usages in early Vedic
