Where does the Japanese Sentence Pattern
"Boku-wa Unagi-da" come from?*

In this paper we have attempted to identify the source sentence of what is called the unagi-bun. After a brief description of typical unagi-bun sentences, we have examined six hypotheses on the source of the unagi-bun. They are the no-da, copula, response, cleft sentence, propredication and ellipsis hypotheses.
We have refuted the no-da hypothesis from the semantic point of view; copula and cleft sentence hypotheses because of their requirement of two variables in the sentence; response hypothesis because of its requirement of a question as a precondition for the existence of the unagi-bun; and propredication hypothesis in terms of the boundness of the sentencehood marker da, its emptiness in meaning and its identical behaviour in optionality between the unagi-bun and ordinary da-pattern sentences.
We have then confirmed Kuno's ellipsis hypothesis based on the syntactic asymmetry of su-ru 'do' and da in substituting for active and stative predicates. Finally, we have argued for optionality of da-attachment and proposed this rule as a global convention.
*Research represented iri this paper has been supported by the Japan foundation in terms of a fellowship.