These examples clearly show that not every {XP, YP} structure is stable, contra
Fukui’s claim.
(15) a.
n[v] NPv[V]
V nPφb.
which[vQ]picture that[C, − WH]TP.. .twh...What differentiates these “unstable” {XP, YP} structures from “stable” ones
like (9b) and (10b)? The answer lies in the existence of Agree(ment), which
can be easily seen by comparing (10b) and (15b) (see also Fukui 2011 for a
different approach). The stable structure in (10b) is minimally different from
the unstable SO in (15b), in that only the former accompanies Q-feature-
agreement. Thus, the relevant {XP, YP} structure becomes stable and “sym-
metric” if there exists Q-feature-agreement between C and wh (as in (10b)),
while it becomes unstable if no Q-feature-agreement is involved (as in (15b)).
The observation that {XP, YP} without agreement is unstable can be directly
extended to cases like (15a) as well. By the standard assumption, n and v are
as distinct in featural contents as they can be, and indeed, no agreement relation
is established between n(P) and v(P). Thus, it is tempting to hypothesize that
the relevant sort of feature-agreement is a necessary condition for making XP-YP
structures symmetric and stable.
To maintain the preliminar y generalization in (11) in its essence while taking
feature-agreement into consideration, we would like to propose the following
concept of feature-equilibrium in (16) and advocate a new characterization of
symmetry (17) in terms of this new concept.
(16) Feature-equilibrium:
For a formal feature F, an SO {α, β} is in an F-equilibrium. ≡def. α and β
share a matching formal feature F that is equally prominent in α and β.
(17) Featural symmetry:
Given a formal feature F in an SO {α, β}, the SO is symmetric with respect
to F (or F-symmetric) if it is in an F-equilibrium. Otherwise, the SO {α, β}
is (F-)asymmetric with respect to F.
According to the proposed definition, {XP, YP} counts as symmetric when it
forms an F-equilibrium with respect to some formal feature F. Familiar XP-YP
16 Hiroki Narita and Naoki Fukui