Jiří Matela’s Heritage of Mikami Akira: A Note on Linguistic Typology is a nice, short introduction to its subject. Mikami Akira 三上章 was a Japanese linguist who created the sentence Zō wa hana ga nagai (“elephant TOPIC nose ?SUBJ/NOM long,” “Elephants have long trunks”). I say ?SUBJ/NOM because, while these are the two analyses typically applied to this sentence in the English literature, well…
Mikami’s objection to the concept of subject in Japanese is a terminological one. Mikami refuses to use the word “subject” for something that does not correspond to the concept of subject in the western linguistics, where the concept originated. […] Mikami’s position is to associate the subject with the noun phrase in the nominative case. However, a nominative noun phrase should control the finite verb and cause a grammatical agreement, the way we know it from European accusative languages, to be recognized as the subject. In Japanese, there is no such morphological agreement, therefore there is no nominative case, and therefore there is no subject.
If we take a look at Mikami’s emblematic sentence, Zō wa hana ga nagai, or “Elephants have long trunks”, we can see Mikami’s point. There is no overt agreement between the predicate adjective nagai, “long”, and either of the two noun phrases (zō wa and hana ga). The word hana does not control the predicate, it is merely semantically connected to the stem of the predicative. The word zō, on the other hand, is the sentence topic, since it is a result of the “topicalization”, a transformation that raises a deep structure genitive noun phrase (zō no) into the sentence topic, marked with the particle wa. Although from the point of view of e.g. construction grammars the notion of transformation (topicalization etc.) is quite problematic, for Mikami, to put it simple, if there is no structural justification to call a noun phrase a subject, there is no reason to ever introduce the concept of subject into the description of the Japanese grammar.
The obvious question: If hana ga isn’t a subject, what is it? Matela doesn’t explore this, but as I understand it (I don’t think I’ve ever read one of Mikami’s own books, shamefully enough), Mikami argues that NPs like hana ga are “subject[ive] complements” (shukaku hogo 主格補語). Might sound like splitting hairs, but that’s science, I guess.