Following the spirit, but not the same mechanism, of Kayne's (1998, 2000) overt syntactic derivational analysis of focus-related elements such as only, also/too, and even in English, this paper explores the nature of focus feature organization in narrow syntax (NS) with special reference to the additive use of the focus particle mo 'also' in Japanese. First, it is argued that, in spite of their attractiveness, at least the versions of LF focus particle movement analysis of association with focus entertained in Aoyagi (1998, 1999, 2006) are faced with theoretical/empirical problems. Then, it is claimed that association with focus in Japanese should be best captured in overt syntax without recourse to any LF movement operations, while keeping to Aoyagi's (1998, 1999, 2006) original insight of taking association with focus as "focus feature agreement or sharing."In so doing, a new hypothesis for focus feature organization in NS (Focus Feature-Splitting Hypothesis) is proposed, which dictates that in principle an interpretable focus feature [focus] and an uninterpretable focus feature [uFoc] in the domain of the Foc head occupied by mo can be separated under some "locality condition"based on phases. To the extent that the proposed overt syntactic derivational approach is on the right track, it provides another empirical support for the single-cycle computational system for NS in the faculty of language (FL), entertained in Kayne (1998, 2000), Epstein et al. (1998), Chomsky (2001b, 2004), and Epstein and Seely (2006) inter alia.
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